materials Communication
Enhanced Interface Structure and Properties of Titanium Carbonitride-Based Cermets with the Extra Solid Phase Reaction Nan Lin 1, *, Yuehui He 2, * and Xiyue Kang 2 1 2
*
College of Materials Science and Engineering, Hunan University, Changsha 410082, China State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China; [email protected] Correspondence: [email protected] (N.L.); [email protected] (Y.H.); Tel.: +86-073-188-877-391 (N.L.); +86-073-188-836-144 (Y.H.); Fax: +86-073-188-877-864 (N.L.); +86-073-188-710-855 (Y.H.)
Received: 15 July 2017; Accepted: 13 September 2017; Published: 15 September 2017
Abstract: In this paper, the influence of the extra solid phase reaction on the interface structure and mechanical properties of titanium carbonitride-based cermets were investigated. The extra solid phase reaction in the preparation process of cermets could induce the formation of a core/rim/binder interface with the coherent structure and reinforce the interface bonding strength in cermets. The existence of a coherent structure interface can inhibit crack spread and improve the toughness and abrasion resistance of titanium carbonitride-based cermets significantly. Cermets can exhibit the high hardness Rockwell Hardness A (HRA) 92.3, fracture toughness of 11.6 MPa·m1/2 , and transverse rupture strength of 2810 MPa. Keywords: cermets; transmission electron microscopy; interface structure; mechanical properties; abrasion resistance
1. Introduction Owing to high hardness, perfect abrasive resistance, excellent oxidation resistance, and low friction coefficient to steel and cast iron, Ti(C,N)-based cermets are widely used in the high-speed cutting of metallic materials, which are composed of Ti(C,N) ceramic particles and a Co/Ni binder phase [1–4]. The inferior toughness of Ti(C,N) cermets, which is caused by the poor wettability between Ti(C,N) particles and the Co/Ni binder phase, can limit the application field of Ti(C,N) cermets [5,6]. The addition of carbides can induce the formation of a rim phase between Ti(C,N) particles and the Co/Ni binder phase, which can improve the wettability and strengthen the bonding between the Ti(C,N) and the Co/Ni binder phase. However, during the conventional vacuum liquid-phase sintering process, the rim phase can cause internal stress due to the mismatch between core and rim lattices, which may bring about a sharp decreasing of mechanical properties in Ti(C,N) cermets, cause the breakage of cermets tools in cutting processes, and damage the lifetime of Ti(C,N)-based cermet tools inevitably [7–9]. The mechanical properties of Ti(C,N)-based cermets are influenced by the sintering densification process, which can determine the interface structure of the core/rim/binder in ceramics. For Ti(C,N)-based cermets, the detailed studies of crystallographic characterizations for core/rim grain boundaries show that the interface structure of core/rim grain boundaries is an incoherent interface [10]. However, a coherent core/rim interface can be found in (Ti,Ta)(C,N)-based cermets developed by a mechanically induced, self-sustaining reaction [11]. However, there are few studies on the effect mechanism among the sintering process, interface structure feature, and mechanical properties of Ti(C,N)-based cermets. Herein,
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we report an innovative method combining a solid phase reaction and a liquid phase sinter-hot isostatic pressing (HIP) to prepare Ti(C,N)-based cermets with a coherent interface in core/rim/binder phase grain boundaries, and illuminate a reasonable mechanism among interface structure, mechanical properties, and abrasion resistance. 2. Experimental Procedures The Ti(C0.5 ,N0.5 )-20 wt %WC-8 wt %Mo2 C-4 wt %TaC-9 wt %Ni-9 wt %Co cermets were prepared in the present work. Ti(C0.5 ,N0.5 ) (2.5 µm), WC (1.2 µm), Mo2 C (1.5 µm), TaC (1.8 µm), Co (0.8 µm), and Ni (1.2 µm) powders were used as raw materials, which were milled by a planetary high-energy ball-mill (XQM-2, Kexi, Nanjing, China) in methanol for 48 hours at the milling speed of 250 r/min and with the ball-to-powder weight ratio of 6:1. Subsequently, the powders were mixed with Materials 2017, 10, 1090 2 of 7 3 wt % paraffin, granulated, and compressed into a rectangular plate under the pressure of 200 MPa. The sintering and a liquid phase sinter-hot isostatic pressing (HIP) to prepare Ti(C,N)-based cermets with a 2 Pa and then heated up to 1300 ◦ C for 120 min to was carried outcoherent in a vacuum with 1 ×grain 10− interface in furnace core/rim/binder phase boundaries, and illuminate a reasonable mechanism among interface structure, mechanical properties, and abrasion resistance. conduct the extra solid phase reaction. Then, sintering was conducted by a sinter-HIP at 1510 ◦ C for 3 60 min in argon2.with an air-pressure Experimental Procedures of 5 MPa. Cermets with dimensions of 20 × 6.5 × 5.25 mm were prepared for microstructural analysis and mechanical properties measurement. The Ti(C0.5,N0.5)-20 wt %WC-8 wt %Mo 2C-4 wt %TaC-9 wt %Ni-9 wt %Co cermets were prepared in the present work. Ti(C 0.5 ,N 0.5 ) (2.5 μm), WC (1.2 μm), Mo2C was (1.5 μm), TaC (1.8 μm),by Co (0.8 The microstructure observation of densified cermets performed an μm), FEI NanoSEM230 and Ni (1.2 μm) powders were used as raw materials, which were milled by a planetary high-energy scanning electron microscope (SEM) (Thermo Fisher Scientific, Hillsboro, OR, USA). Transmission electron ball-mill (XQM-2, Kexi, Nanjing, China) in methanol for 48 hours at the milling speed of 250 r/min and with the ball-to-powder ratio of 6:1. the powders were mixed withJapan). 3 wt % Hardness was microscopy (TEM) observations wereweight conducted in Subsequently, JEM-2100-200 kv (JEOL, Tokyo, paraffin, granulated, and compressed into a rectangular plate under the pressure of 200 MPa. tested by a Rockwell hardness tester (INSTRON, Boston, MA, USA) under a constant load of 60 kg as well The sintering was carried out in a vacuum furnace with 1 × 10−2 Pa and then heated up to 1300 °C for as a Vickers hardness (Huayin Ltd., Laizhou, under 120 min tester to conduct the extra Testing solid phaseInstrument reaction. Then,Co., sintering was conducted China) by a sinter-HIP at a constant load 3 1510 °C for 60 rupture min in argonstrength with an air-pressure of 5 MPa. Cermets with dimensions of 20 × 6.5with × 5.25 mm of 30 kg. The transverse was measured at room temperature a loading speed of were prepared for microstructural analysis and mechanical properties measurement. 2 mm/min by the three-point bending technique with INSTRON 3369 Boston, MA, USA). The microstructure observation of densified cermets was performed by(INSTRON, an FEI NanoSEM230 electronfor microscope (SEM) rupture (Thermo Fisher Scientific, Hillsboro, OR, toughness USA). Transmission Five specimensscanning were tested transverse strength. The fracture was determined by electron microscopy (TEM) observations were conducted in JEM-2100-200 kv (JEOL, Tokyo, Japan). measuring the length of cracks in the Vickers indentation and calculations with the Shetty formula [12]. Hardness was tested by a Rockwell hardness tester (INSTRON, Boston, MA, USA) under a constant load of 60 kgof as cermets well as a Vickers tester Testing reciprocating Instrument Co., Ltd., Laizhou,testing machine The abrasion resistance was hardness tested in a (Huayin high-speed friction China) under a constant load of 30 kg. The transverse rupture strength was measured at room (HRS-2M, Zhongkekaihua, Lanzhou, China) for 10 min at the applied load of 50 N by sliding against temperature with a loading speed of 2 mm/min by the three-point bending technique with INSTRON a WC-6Co ball with a rotating speed ofUSA). 300 r/min. 3369 (INSTRON, Boston, MA, Five specimens were tested for transverse rupture strength. The fracture toughness was determined by measuring the length of cracks in the Vickers indentation and calculations with the Shetty formula [12]. The abrasion resistance of cermets was tested in a high3. Result and Discussion
speed reciprocating friction testing machine (HRS-2M, Zhongkekaihua, Lanzhou, China) for 10 min
at the applied load of 50 N by sliding against a WC-6Co ball with a rotating speed of 300 r/min. The microstructures of Ti(C,N)-based cermets, which were synthesized by the extra solid phase reaction and sinter-HIP, are shown in Figure 1. Figure 1a shows a typical core-rim-binder microstructure 3. Result and Discussion in Ti(C,N)-based cermets. The black core phases arewhich Ti(C,N) particles,bywhich not dissolved and The microstructures of Ti(C,N)-based cermets, were synthesized the extrawere solid phase reaction and sinter-HIP, are shown in Figure 1. Figure 1a shows a typical core-rim-binder reacted in the sintering process. Moreover, the gray rim phases are the solid solutions, which are microstructure in Ti(C,N)-based cermets. The black core phases are Ti(C,N) particles, which were not composed of (Ti,W,Mo,Ta)(C,N) [13,14]. The white binder phase is observed along the rim phase, dissolved and reacted in the sintering process. Moreover, the gray rim phases are the solid solutions, are composed (Ti,W,Mo,Ta)(C,N) white binder is observedproperties. along the rim which can bringwhich about a largerofmean free path[13,14]. and The improve thephase mechanical phase, which can bring about a larger mean free path and improve the mechanical properties.
Figure 1. SEM micrographs (a) and TEM observation (b) of the microstructure in Ti(C,N)-based cermets prepared (a) by solid reaction and liquid phase Figure 1. SEM micrographs andphase TEM observation (b) ofsinter-HIP. the microstructure in Ti(C,N)-based cermets prepared by solid phase reaction and liquid phase sinter-HIP.
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The measured properties of Ti(C,N)-based cermets are listed in Table 1, together with comparisons from the literature. The hardness, transverse rupture strength, and fracture toughness of cermets prepared by2017, the10,solid Materials 1090 phase reaction and liquid phase sinter-HIP were measured as 92.3 3 of 7HRA, 2810 MPa, and 11.6 MPa·m1/2 , respectively (see more transverse rupture strength details in Figure S1 The measured properties of Ti(C,N)-based cermets are listed in Table together withphase comparisons in supplementary data). The relative densities of cermets prepared by1,direct liquid sinter-HIP, from the literature. The hardness, transverse rupture strength, and fracture toughness of cermets and by solid phase reaction and liquid phase sinter-HIP were 99.6% and 99.8%, which means that the by the solid phase reaction and liquid phase sinter-HIP were measured as 92.3 HRA, extraprepared solid phase reaction can enhance the densification process, reduce the porosity, and improve 2810 MPa, and 11.6 MPa·m1/2, respectively (see more transverse rupture strength details in Figure S1 the transverse rupture strength of cermets. Compared with the values of mechanical properties in in supplementary data). The relative densities of cermets prepared by direct liquid phase sinter-HIP, Ti(C,N)-based cermets manufactured by the sinter-HIP, vacuum sintering, and hot pressing [10,15,16], and by solid phase reaction and liquid phase sinter-HIP were 99.6% and 99.8%, which means that the the present Ti(C,N)-based cermets exhibited an excellentprocess, mechanical properties. Therefore, the extra extra solid phase reaction can enhance the densification reduce the porosity, and improve solid the phase reaction can improve mechanical properties of Ti(C,N)-based cermets, especially transverse rupture strength the of cermets. Compared with the values of mechanical properties in the transverse rupturecermets strength. Ti(C,N)-based manufactured by the sinter-HIP, vacuum sintering, and hot pressing [10,15,16], the present Ti(C,N)-based cermets exhibited an excellent mechanical properties. Therefore, the extra Tablereaction 1. Properties of Ti(C,N) prepared by solid phase reactioncermets, and sinter-HIP. solid phase can improve thecermets mechanical properties of Ti(C,N)-based especially the transverse rupture strength. Specimen
Density
Relative
Specimen Present work
Present work [10] Present [15] work [16][10] [15] [16]
solid phase Sintering Density 3) Method reaction + liquid (g/cm7.120 phase sinter-HIP solid phase liquid phase 7.120 reaction + liquid 7.105 sinter-HIP phase sinter-HIP vacuum sintering – liquid phase vacuum sintering 7.105 – sinter-HIP hot pressing – vacuum sintering -vacuum sintering hot pressing
---
Relative Density 99.8 (%)
Rockwell A Hardness 92.3
99.899.6
92.3 92.2
–
92.2
99.6 –
92.2 90.3
----
–
(MPam1/2 )
(GPa)
(%)
Present work
Fracture
Vickers
Rockwell
Density Sintering Method of Ti(C,N) Table 1. Properties by solid phaseHardness reaction andToughness sinter-HIP. A Hardness (g/cm3 ) cermets prepared
92.2 90.3 91.0
91.0
Transverse Rupture Strength (MPa)
Transverse Vickers Fracture Rupture Hardness Toughness 1680 ± 20 11.6 ± 0.3 2810 ± 80 Strength (MPa) (GPa) (MPam1/2) 1680 ± 1610 20 ± 10 11.6 ± 0.3 9.6 ± 0.2
–
2810 1820 ± 80 ± 70
9.2
1804
1610 ± 10 1380 9.6 ± 0.2 10.2
1820 ± 70 1505
-1380 1500
1500
9.2 10.2 8.3
8.3
1804 1505 1200
1200
Figure 2 shows the crack propagation of Ti(C,N)-based cermets. The indentation crack is extended along the Figure grain boundary in cermet prepared byofdirect liquid phase sinter-HIP, and the Ti(C,N) grains 2 shows the crack propagation Ti(C,N)-based cermets. The indentation crack is extended along grain boundary in cermet prepared by direct liquid phase2a. sinter-HIP, the that cannot influence thethe propagation of crack path distinctly, as shown in Figure Figure 2band shows Ti(C,N) grains deflection cannot influence the propagation crack path distinctly, as and shown in Figurechanged 2a. the mode of crack in cermet prepared byofthe solid phase reaction sinter-HIP Figure 2b shows that the mode of crack deflection in cermet prepared by the solid phase reaction and into a trans-granular fracture. This hints that the excellent bond strength of the core/rim/binder changed into a trans-granular This hintsthe thatcrack the excellent strength the grainsinter-HIP boundary in Ti(C,N)-based cermetsfracture. may influence spread bond in the crack of extension core/rim/binder grain boundary in Ti(C,N)-based cermets may influence the crack spread in the crack direction, generate an amount of resistance to the crack propagation, and enhance the toughness of extension direction, generate an amount of resistance to the crack propagation, and enhance the cermets drastically. toughness of cermets drastically.
(a)
(b) 2 μm
intergranular fracture
2 μm
transgranular fracture
5 μm
5 μm
Figure 2. SEM micrographs of crack with Ti(C,N)-based cermets prepared by direct liquid phase
Figure 2. SEM micrographs of crack with Ti(C,N)-based cermets prepared by direct liquid phase sinter-HIP (a), and solid phase reaction and liquid phase sinter-HIP (b). sinter-HIP (a), and solid phase reaction and liquid phase sinter-HIP (b).
The wear properties and compositions of abrasive dust of Ti(C,N)-based cermets with the different processes are shown in Figure 3. Figure 3aof shows that the extra solid phase The wearsintering properties and compositions of abrasive dust Ti(C,N)-based cermets with reaction the different can contribute to the reduction of the depth value of the wear track and wear volume in the process can sintering processes are shown in Figure 3. Figure 3a shows that the extra solid phase reaction of abrasion. It can be seen from Figures 3b,c that the W element contents of the abrasive dust are
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of the depth value of the wear track and wear volume in the process 4 of 7 of abrasion. It can be seen from Figure 3b,c that the W element contents of the abrasive dust4 are Materials 2017, 10, 1090 of 7 26.87 wt wt % % and and41.18 41.18wt wt% %in inthe theTi(C,N)-based Ti(C,N)-based cermets cermets with with the the extra extra solid solid phase phase reaction reaction and and direct direct 26.87 liquid phase sinter-HIP. cermets with solid phase reaction 26.87 wt % and 41.18 wt % in the Ti(C,N)-based cermets with the extra phase reaction and direct liquid phase sinter-HIP. This means that Ti(C,N)-based cermets with the extra solid phase reaction can possess higher abrasion resistance and achieve a larger quality of W element from the WC-Co liquid phasehigher sinter-HIP. Thisresistance means that Ti(C,N)-based cermets with extra solid phase can possess abrasion and achieve a larger quality ofthe W element from the reaction WC-Co ball in the the wear wear track cermets. According to Table Table and quality Figure 2, 2, the solid phase phase reaction and can in possess higher abrasion resistance and achieve a larger of the W element fromreaction the WC-Co ball track of cermets. According to 11 and Figure solid and sinter-HIP can bring about the higher transverse rupture strength and fracture toughness of cermets, ball in the wear track of cermets. According to Table 1 and Figure 2, the solid phase reaction and sinter-HIP can bring about the higher transverse rupture strength and fracture toughness of cermets, can the outstanding bond strength ofrupture theofinterface among grain boundaries and improve sinter-HIP can bring about the higher transverse strength andamong fracture toughness of cermets, which caninduce induce the outstanding bond strength the interface grain boundaries and the abrasion resistance. which can the outstanding bond strength of the interface among grain boundaries and improve theinduce abrasion resistance. improve the abrasion resistance.
Figure 3. The wear depth of cermets (a) and SEM images of the wear track for cermets with the direct Figure 3. The wear depth of cermets (a) and SEM images of the wear track for cermets with the direct liquid (b)ofand extra(a) solid (c). Figurephase 3. Thesinter-HIP wear depth cermets andphase SEM reaction images of the wear track for cermets with the direct liquid phase sinter-HIP (b) and extra solid phase reaction (c). liquid phase sinter-HIP (b) and extra solid phase reaction (c).
To investigate the morphology and structure of the cermets in depth, HRTEM of cermets were To morphology and structure depth, HRTEM of were To investigate investigate the morphology andmicrograph structure of ofofthe the cermets inand depth, HRTEM of cermets cermets were performed. Figure 4the shows the HRTEM thecermets core/rimin rim/binder grain boundaries, performed. Figure 4 shows the HRTEM micrograph of the core/rim and rim/binder grain boundaries, performed. 4 shows the HRTEM micrograph of thein core/rim andItrim/binder grain boundaries, and the FastFigure Fourier Transformation patterns are shown the insets. can be seen from Figure 4a and the Fast Fourier Transformation are shown It can can becore seenand from 4a and the thecore Fastand Fourier Transformation patterns shown in the insets. It be seen from Figure 4a that rim grain of preparedpatterns cermetsare match wellin atthe the insets. boundary. The rimFigure crystals that the core and rim grain of prepared cermets match well at the boundary. The core and rim crystals that the core and rim grain of prepared cermets match well at the boundary. The core and rim crystals have no spatial orientation difference or obvious lattice distortion. The measured interplanar have difference orrim obvious lattice distortion. The respectively, measured interplanar distances have no nospatial spatial orientation difference or distortion. The measured distances for theorientation (−11−1) core and (−11−1) are obvious 0.248 nmlattice and 0.252 nm, which interplanar reveals the for the ( − 11 − 1) and ( − 11 − 1) are 0.248 nm and 0.252 nm, respectively, which reveals the high core rim lattice distances for theof (−11−1) core and (−11−1) rim are 0.248 nmisand 0.252The nm,excellent respectively, whichofreveals the high coherence the interface (the mismatch 0.98%). matching neighbor coherence of the interface (the lattice mismatch is 0.98%). The excellent matching of neighbor crystal high coherence the interface (theboundary lattice mismatch 0.98%). The interface excellentstability matching ofimproves neighbor crystal lattices atof the core/rim grain increasesisthe core/rim and lattices atstrength the core/rim grain boundary increases core/rim interface stability and and improves the crystal lattices at the core/rim grain boundary the core/rim stability improves the bond of the grain boundary. Figureincreases 4b the shows that there isinterface a excellent match between rim bond strength of the grain boundary. Figure 4b shows that there is a excellent match between rim the bond strength boundary. Figure shows thatphase there have is a excellent match between rim and binder phasesof atthe thegrain interface. The rim grain4band binder an orientation relationship and binder phases at the interface. The rim grain and binder phase have an orientation relationship and(200) binder phases at the interface. The rim grain and binderdistances phase have orientation of rim//(11−1) binder at the interface, whose interplanar are an 0.221 nm andrelationship 0.216 nm, of //(11 −binder 1)isbinder at the interface, whose interplanar distances are 0.221 nmand andThe 0.216 nm, of (200) (200)rim rim//(11−1) at the interface, interplanar distances are nm 0.216 nm, respectively. There the misfit of 2.3% whose between rim and binder phase at0.221 the interface. lattice respectively. Therethe is the misfit 2.3% between and binder phase at the interface. The respectively. There is the misfit of ofphase 2.3%boundary between rim rim and the binder phase atand the binder interface. The lattice lattice distortion between rim/binder reduces misfit of rim crystals along distortion between the rim/binder phase boundary reduces the misfit of rim and binder crystals along distortion thebinder rim/binder phase boundary reduces the misfit of rim/binder rim and binder along the (200)rimbetween and (11−1) plane and enhances the bond strength of the graincrystals boundary. the (200) and (11 − 1) plane and enhances the bond strength of the rim/binder grain boundary. binder the (200)rim rim and (11−1)binder plane and enhances the bond strength of the rim/binder grain boundary.
Figure 4. Cont.
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Figure 4. HRTEM micrograph of the core/rim interface (a) (corresponding to region A in Figure 1) Figure 4. HRTEM micrograph of the core/rim interface (a) (corresponding to region A in Figure 1) and andrim/binder the rim/binder phase interface in prepared cermets (b) (corresponding to region in Figure the phase interface in prepared cermets (b) (corresponding to region B in B Figure 1). 1).
In the sintering process of Ti(C,N)-based cermets, Ti(C,N) particles can dissolve in the binder sintering processand of Ti(C,N)-based cermets,could Ti(C,N) particles can dissolve in the phaseInatthe high temperatures, Ti, C, and N elements reprecipitate as Ti(C,N) from the binder binder phase at high temperatures, and Ti, C, and N elements could reprecipitate as Ti(C,N) from the binder phase. The addition of carbides can precipitate as the rim phases in the form of a complicated phase. The solution. addition During of carbides can precipitate assintering the rim process, phases in of amay complicated (Ti,M)(C,N) the direct liquid phase thethe fastform heating cause an (Ti,M)(C,N) solution. During the direct liquid phase sintering process, the fast heating may cause unordered formation of the microstructure, induce the generation of core/rim/binder interfaces with an unordered formation of the microstructure, induce the generation of core/rim/binder interfaces incoherent structure, and form a Ti(C,N)/binder grain boundary with weak bond strength, which can with structure, and form a Ti(C,N)/binder grain boundarydirect with liquid weak phase bond sintering strength, causeincoherent a decrease in the toughness [10,17]. Compared to the conventional which can cause a decrease in the toughness [10,17]. Compared to the conventional direct liquid phase method for controlling the formation of core/rim structure in cermets [9,10], the solid phase reaction sintering method controlling the formation of core/rim structure cermets [9,10], the solid and liquid phasefor sinter-HIP method can solve the problem of a in lack of bond strength at phase grain reaction and liquid phase sinter-HIP method can solve the problem of a lack of bond strength atat grain boundaries. During the sintering process, the formation of the rim structure can be achieved the boundaries. During the sintering process, the formation of the rim structure can be achieved at the solid stage sintering. Mo2C and TaC can react with Ti(C,N) particles and form the rim phase during solid stage sintering. Mobefore TaC °C. canThe reactsolid withstage Ti(C,N) particles form the the diffusion rim phasereaction during 2 C and1300 the solid stage sintering sintering canand enhance ◦ C. The solid stage sintering can enhance the diffusion reaction the solid stage sintering before 1300 between WC and Ti(C,N) particles, and form the rim phase (see Figures S2 and S3 in supplementary between WC and the Ti(C,N) particles, form thethe rimcrystal phase (see Figures S2 and S3 in supplementary data). Moreover, rim phase canand grow along structure of Ti(C,N) core by an oriented data). Moreover, the rimand phase can grow along theconsistency crystal structure of Ti(C,N) core bywith an oriented attachment mechanism, achieve the structural of the core/rim interface various attachment mechanism, and achieve the structural consistency of the core/rim interface with various constituents. Moreover, in the liquid phase sintering process, the dissolution of (Ti,M)(C,N) from the constituents. Moreover, in the liquid phase sintering process, the dissolution of (Ti,M)(C,N) from the Ti(C,N)/(Ti,M)(C,N) core/rim grains can generate a (Ti,M)(C,N)-rich layer at the interface between Ti(C,N)/(Ti,M)(C,N) core/rim grains candifference generate a layerinterface at the interface between (Ti,M)(C,N) and the binder phase. The in(Ti,M)(C,N)-rich the core/rim/binder relationships of (Ti,M)(C,N) and the binder phase. The difference in the core/rim/binder interface relationships of cermets fabricated by diverse sintering methods can cause variation in the toughness. It is well known cermets fabricated by diverse sintering methods cause variation in the It is well known that the transverse rupture strength and fracturecan toughness of cermets aretoughness. strongly dependent on the that the transverse rupture strength and fracture toughness of cermets are strongly dependent the interface characteristics between the metal matrix and ceramic particles [18]. Toughening viaon crack interface characteristics between the metal matrix and ceramic particles [18]. Toughening via crack deflection can be established for brittle materials [19], and the strain energy release rate of cermets, deflection be established forthe brittle materials and the strain energy release rate ofstructure cermets, Gcermet, can can be estimated through brittle fracture[19], of Ti(C,N)/(Ti, W, Ta, Mo)(C,N) core/rim G , can be estimated through the brittle fracture of Ti(C,N)/(Ti, W, Ta, Mo)(C,N) core/rim structure cermet and the plastic rupture of (Co,Ni). The combined effects of the core/rim structure and Co/Ni binder and the plastic rupture can be described by: of (Co,Ni). The combined effects of the core/rim structure and Co/Ni binder can be described by: Gcermet (1 V − Vbinder)G )Gcore-rim core-rim + Vbinderσodbinderχ (1) Gcermet = (1= − + Vbinder σo dbinder χ (1) binder where G Gcore-rim isisthe release rate rateofofthe thecore-rim core-rimstructure, structure, bulk flow stress of where thestrain strain energy energy release σoσiso is thethe bulk flow stress of the core-rim the binder, χ is the function bond strength in the rim/binder χ canwith increase with binder, and χand is the function of bondofstrength in the rim/binder interface.interface. χ can increase increasing increasing the bond strength of the interface. The drastically reduced lattice misfit at the the bond strength of the interface. The drastically reduced lattice misfit at the core/rim/binder core/rim/binder interfaces stabilize the coherent interface lowering elastic phase interfaces phase can stabilize the can coherent interface by lowering the by elastic strain the energy [20]strain and energy [20] and improve the bond strength of the grain boundary, which can enhance the mechanical improve the bond strength of the grain boundary, which can enhance the mechanical properties and properties and abrasion resistance. Therefore, Ti(C,N)-based prepared by thereaction extra solid abrasion resistance. Therefore, Ti(C,N)-based cermets preparedcermets by the extra solid phase can phase reaction can possess a core/rim/binder grain boundary with coherent structure, which can possess a core/rim/binder grain boundary with coherent structure, which can enhance the mechanical enhance the mechanical properties and abrasion resistance of the Ti(C,N)-based cermets. properties and abrasion resistance of the Ti(C,N)-based cermets.
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4. Conclusions Ti(C,N)-based cermets with ultrahigh transverse rupture strength can be prepared by the solid phase reaction and liquid phase sinter-HIP technique. It was found that the core/rim/binder grains can have a coherent interface structure at the grain boundary, which can be favorable to enhance the mechanical properties and improve the abrasion resistance of Ti(C,N)-based cermets. The successful synthesis of the coherent phase boundary should be suitable for the manufacture of high-quality Ti(C,N)-based cermets, and provide a novel approach to achieve excellent properties in cermets. Supplementary Materials: The following are available online at www.mdpi.com/1996-1944/10/9/1090/s1, Figure S1: The curve of transverse rupture strength for cermets prepared by the extra solid phase, Figure S2: SEM micrographs of microstructure in Ti(C,N)-based cermets sintered at 1300 ◦ C for 0 h (a) and 2 h (b), Figure S3: XRD patterns of Ti(C,N)-based cermets sintered at 1300 ◦ C for 0 h and 2 h. Acknowledgments: This work is supported by Research Funds for Central Universities (531107040967) and National Natural Science Foundation of China (No. 51634006). Author Contributions: Nan Lin and Yuehui He conceived of and designed the experiments. Nan Lin performed the experiments. Nan Lin and Xiyue Kang analyzed the data. Nan Lin and Yuehui He wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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Wang, H.; Zhu, W.C.; Liu, Y.C.; Zeng, L.K.; Sun, L.Y. The Microwave-Assisted Green Synthesis of TiC Powders. Materials 2016, 9, 904. [CrossRef] [PubMed] Wan, W.; Xiong, J.; Liang, M. Effects of secondary carbides on the microstructure, mechanical properties and erosive wear of Ti(C,N)-based cermets. Ceram. Int. 2017, 43, 944–952. [CrossRef] Chicardi, E.; Gotor, F.J.; Córdoba, J.M. Enhanced oxidation resistance of Ti(C,N)-based cermets containing Ta. Corros. Sci. 2014, 84, 11–20. [CrossRef] Córdoba, J.M.; Chicardi, E.; Gotor, F.J. Liquid-phase sintering of Ti(C,N)-based cermets. The effects of binder nature and content on the solubility and wettability of hard ceramic phases. J. Alloys Compd. 2013, 559, 34–38. [CrossRef] Kim, S.; Min, K.; Kang, S. Rim structure in Ti(C0.7 N0.3 )-WC-Ni cermets. J. Am. Ceram. Soc. 2003, 86, 1761–1766. [CrossRef] De la Obra, A.G.; Gotor, F.J.; Chicardi, E. Effect of the impact energy on the chemical homogeneity of a (Ti,Ta,Nb)(C,N) solid solution obtained via a mechanically induced self-sustaining reaction. J. Alloys Compd. 2017, 708, 1008–1017. [CrossRef] Liu, Y.; Jin, Y.Z.; Yu, H.J.; Ye, J.W. Ultrafine (Ti,M)(C,N)-based cermets with optimal mechanical properties. Int. J. Refract. Met. Hard Mater. 2011, 29, 104–107. [CrossRef] Xu, Q.Z.; Zhao, J.; Ai, X. Fabrication and cutting performance of Ti(C,N)-based cermet tools used for machining of high-strength steels. Ceram. Int. 2017, 43, 6286–6294. [CrossRef] Ortner, H.M.; Ettmayer, P.; Kolaska, H.; Smid, I. The history of the technological progress of hardmetals. Int. J. Refract. Met. Hard Mater. 2015, 49, 3–8. [CrossRef] Shi, Z.M.; Yin, D.Z.; Zhang, D.Y.; Liu, X.W. Characterisation of Ti(C,N)-based cermets with various nitrogen contents studied by EBSD/SEM and TEM. J. Alloys Compd. 2017, 695, 2857–2864. [CrossRef] Chicardi, E.; Córdoba, J.M.; Sayagués, M.J.; Gotor, F.J. Inverse core-rim microstructure in (Ti,Ta)(C,N)-based cermets developed by a mechanically induced self-sustaining reaction. Int. J. Refract. Met. Hard Mater. 2012, 31, 39–46. [CrossRef] Shetty, D.K.; Wright, I.G.; Mincer, P.N.; Clauer, A.H. Indentation fracture of WC-Co cermets. Mater. Sci. 1985, 20, 1873–1882. [CrossRef] Zhao, Y.J.; Zheng, Y.; Zhou, W.; Zhang, J.; Huang, Q.; Xiong, W.H. Effect of carbon addition on the densification behavior, microstructure evolution and mechanical properties of Ti(C,N)-based cermets. Ceram. Int. 2016, 42, 5487–5496. [CrossRef] Naidoo, M.; Johnson, O.; Sigalas, I.; Herrmann, M. Influence of tantalum on the microstructure and properties of Ti(C,N)-Ni cermets. Int. J. Refract. Met. Hard Mater. 2014, 42, 97–102. [CrossRef]
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Xu, Q.Z.; Ai, X.; Zhao, J.; Zhang, H.; Qin, W.; Gong, F. Effect of heating rate on the mechanical properties and microstructure of Ti(C,N)-based cermets. Mater. Sci. Eng. A 2015, 628, 281–287. [CrossRef] Xu, Q.Z.; Ai, X.; Zhao, J.; Qin, W.; Wang, Y.; Gong, F. Comparison of Ti(C,N)-based cermets processed by hot-pressing sintering and conventional pressureless sintering. J. Alloys Compd. 2015, 619, 538–543. [CrossRef] Xiong, W.H. On the mechanism of phase interface combination in Ti(C,N)-Based cermet. J. Huazhong Univ. Sci. Technol. 1995, 23, 28–32. Song, X.; Gao, Y.; Liu, X.; Wei, C.; Wang, H.; Xu, W. Effect of interface characteristics on toughness of nanocrystalline cemented carbides. Acta Mater. 2013, 61, 2154–2162. [CrossRef] Ravichandran, K.S. Fracture Toughness of Two Phase WC-Co Cermets. Acta Mater. 1994, 42, 143–150. [CrossRef] Wu, G.; Chan, K.; Zhu, L.; Sun, L.; Lu, J. Dual-phase nanostructuring as a route to high strength magnesium alloys. Nature 2017, 545, 80–83. [CrossRef] [PubMed] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Enhanced Interface Structure and Properties of Titanium Carbonitride-Based Cermets with the Extra Solid Phase Reaction Nan Lin 1, *, Yuehui He 2, * and Xiyue Kang 2 1 2
*
College of Materials Science and Engineering, Hunan University, Changsha 410082, China State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China; [email protected] Correspondence: [email protected] (N.L.); [email protected] (Y.H.); Tel.: +86-073-188-877-391 (N.L.); +86-073-188-836-144 (Y.H.); Fax: +86-073-188-877-864 (N.L.); +86-073-188-710-855 (Y.H.)
Received: 15 July 2017; Accepted: 13 September 2017; Published: 15 September 2017
Abstract: In this paper, the influence of the extra solid phase reaction on the interface structure and mechanical properties of titanium carbonitride-based cermets were investigated. The extra solid phase reaction in the preparation process of cermets could induce the formation of a core/rim/binder interface with the coherent structure and reinforce the interface bonding strength in cermets. The existence of a coherent structure interface can inhibit crack spread and improve the toughness and abrasion resistance of titanium carbonitride-based cermets significantly. Cermets can exhibit the high hardness Rockwell Hardness A (HRA) 92.3, fracture toughness of 11.6 MPa·m1/2 , and transverse rupture strength of 2810 MPa. Keywords: cermets; transmission electron microscopy; interface structure; mechanical properties; abrasion resistance
1. Introduction Owing to high hardness, perfect abrasive resistance, excellent oxidation resistance, and low friction coefficient to steel and cast iron, Ti(C,N)-based cermets are widely used in the high-speed cutting of metallic materials, which are composed of Ti(C,N) ceramic particles and a Co/Ni binder phase [1–4]. The inferior toughness of Ti(C,N) cermets, which is caused by the poor wettability between Ti(C,N) particles and the Co/Ni binder phase, can limit the application field of Ti(C,N) cermets [5,6]. The addition of carbides can induce the formation of a rim phase between Ti(C,N) particles and the Co/Ni binder phase, which can improve the wettability and strengthen the bonding between the Ti(C,N) and the Co/Ni binder phase. However, during the conventional vacuum liquid-phase sintering process, the rim phase can cause internal stress due to the mismatch between core and rim lattices, which may bring about a sharp decreasing of mechanical properties in Ti(C,N) cermets, cause the breakage of cermets tools in cutting processes, and damage the lifetime of Ti(C,N)-based cermet tools inevitably [7–9]. The mechanical properties of Ti(C,N)-based cermets are influenced by the sintering densification process, which can determine the interface structure of the core/rim/binder in ceramics. For Ti(C,N)-based cermets, the detailed studies of crystallographic characterizations for core/rim grain boundaries show that the interface structure of core/rim grain boundaries is an incoherent interface [10]. However, a coherent core/rim interface can be found in (Ti,Ta)(C,N)-based cermets developed by a mechanically induced, self-sustaining reaction [11]. However, there are few studies on the effect mechanism among the sintering process, interface structure feature, and mechanical properties of Ti(C,N)-based cermets. Herein,
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we report an innovative method combining a solid phase reaction and a liquid phase sinter-hot isostatic pressing (HIP) to prepare Ti(C,N)-based cermets with a coherent interface in core/rim/binder phase grain boundaries, and illuminate a reasonable mechanism among interface structure, mechanical properties, and abrasion resistance. 2. Experimental Procedures The Ti(C0.5 ,N0.5 )-20 wt %WC-8 wt %Mo2 C-4 wt %TaC-9 wt %Ni-9 wt %Co cermets were prepared in the present work. Ti(C0.5 ,N0.5 ) (2.5 µm), WC (1.2 µm), Mo2 C (1.5 µm), TaC (1.8 µm), Co (0.8 µm), and Ni (1.2 µm) powders were used as raw materials, which were milled by a planetary high-energy ball-mill (XQM-2, Kexi, Nanjing, China) in methanol for 48 hours at the milling speed of 250 r/min and with the ball-to-powder weight ratio of 6:1. Subsequently, the powders were mixed with Materials 2017, 10, 1090 2 of 7 3 wt % paraffin, granulated, and compressed into a rectangular plate under the pressure of 200 MPa. The sintering and a liquid phase sinter-hot isostatic pressing (HIP) to prepare Ti(C,N)-based cermets with a 2 Pa and then heated up to 1300 ◦ C for 120 min to was carried outcoherent in a vacuum with 1 ×grain 10− interface in furnace core/rim/binder phase boundaries, and illuminate a reasonable mechanism among interface structure, mechanical properties, and abrasion resistance. conduct the extra solid phase reaction. Then, sintering was conducted by a sinter-HIP at 1510 ◦ C for 3 60 min in argon2.with an air-pressure Experimental Procedures of 5 MPa. Cermets with dimensions of 20 × 6.5 × 5.25 mm were prepared for microstructural analysis and mechanical properties measurement. The Ti(C0.5,N0.5)-20 wt %WC-8 wt %Mo 2C-4 wt %TaC-9 wt %Ni-9 wt %Co cermets were prepared in the present work. Ti(C 0.5 ,N 0.5 ) (2.5 μm), WC (1.2 μm), Mo2C was (1.5 μm), TaC (1.8 μm),by Co (0.8 The microstructure observation of densified cermets performed an μm), FEI NanoSEM230 and Ni (1.2 μm) powders were used as raw materials, which were milled by a planetary high-energy scanning electron microscope (SEM) (Thermo Fisher Scientific, Hillsboro, OR, USA). Transmission electron ball-mill (XQM-2, Kexi, Nanjing, China) in methanol for 48 hours at the milling speed of 250 r/min and with the ball-to-powder ratio of 6:1. the powders were mixed withJapan). 3 wt % Hardness was microscopy (TEM) observations wereweight conducted in Subsequently, JEM-2100-200 kv (JEOL, Tokyo, paraffin, granulated, and compressed into a rectangular plate under the pressure of 200 MPa. tested by a Rockwell hardness tester (INSTRON, Boston, MA, USA) under a constant load of 60 kg as well The sintering was carried out in a vacuum furnace with 1 × 10−2 Pa and then heated up to 1300 °C for as a Vickers hardness (Huayin Ltd., Laizhou, under 120 min tester to conduct the extra Testing solid phaseInstrument reaction. Then,Co., sintering was conducted China) by a sinter-HIP at a constant load 3 1510 °C for 60 rupture min in argonstrength with an air-pressure of 5 MPa. Cermets with dimensions of 20 × 6.5with × 5.25 mm of 30 kg. The transverse was measured at room temperature a loading speed of were prepared for microstructural analysis and mechanical properties measurement. 2 mm/min by the three-point bending technique with INSTRON 3369 Boston, MA, USA). The microstructure observation of densified cermets was performed by(INSTRON, an FEI NanoSEM230 electronfor microscope (SEM) rupture (Thermo Fisher Scientific, Hillsboro, OR, toughness USA). Transmission Five specimensscanning were tested transverse strength. The fracture was determined by electron microscopy (TEM) observations were conducted in JEM-2100-200 kv (JEOL, Tokyo, Japan). measuring the length of cracks in the Vickers indentation and calculations with the Shetty formula [12]. Hardness was tested by a Rockwell hardness tester (INSTRON, Boston, MA, USA) under a constant load of 60 kgof as cermets well as a Vickers tester Testing reciprocating Instrument Co., Ltd., Laizhou,testing machine The abrasion resistance was hardness tested in a (Huayin high-speed friction China) under a constant load of 30 kg. The transverse rupture strength was measured at room (HRS-2M, Zhongkekaihua, Lanzhou, China) for 10 min at the applied load of 50 N by sliding against temperature with a loading speed of 2 mm/min by the three-point bending technique with INSTRON a WC-6Co ball with a rotating speed ofUSA). 300 r/min. 3369 (INSTRON, Boston, MA, Five specimens were tested for transverse rupture strength. The fracture toughness was determined by measuring the length of cracks in the Vickers indentation and calculations with the Shetty formula [12]. The abrasion resistance of cermets was tested in a high3. Result and Discussion
speed reciprocating friction testing machine (HRS-2M, Zhongkekaihua, Lanzhou, China) for 10 min
at the applied load of 50 N by sliding against a WC-6Co ball with a rotating speed of 300 r/min. The microstructures of Ti(C,N)-based cermets, which were synthesized by the extra solid phase reaction and sinter-HIP, are shown in Figure 1. Figure 1a shows a typical core-rim-binder microstructure 3. Result and Discussion in Ti(C,N)-based cermets. The black core phases arewhich Ti(C,N) particles,bywhich not dissolved and The microstructures of Ti(C,N)-based cermets, were synthesized the extrawere solid phase reaction and sinter-HIP, are shown in Figure 1. Figure 1a shows a typical core-rim-binder reacted in the sintering process. Moreover, the gray rim phases are the solid solutions, which are microstructure in Ti(C,N)-based cermets. The black core phases are Ti(C,N) particles, which were not composed of (Ti,W,Mo,Ta)(C,N) [13,14]. The white binder phase is observed along the rim phase, dissolved and reacted in the sintering process. Moreover, the gray rim phases are the solid solutions, are composed (Ti,W,Mo,Ta)(C,N) white binder is observedproperties. along the rim which can bringwhich about a largerofmean free path[13,14]. and The improve thephase mechanical phase, which can bring about a larger mean free path and improve the mechanical properties.
Figure 1. SEM micrographs (a) and TEM observation (b) of the microstructure in Ti(C,N)-based cermets prepared (a) by solid reaction and liquid phase Figure 1. SEM micrographs andphase TEM observation (b) ofsinter-HIP. the microstructure in Ti(C,N)-based cermets prepared by solid phase reaction and liquid phase sinter-HIP.
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The measured properties of Ti(C,N)-based cermets are listed in Table 1, together with comparisons from the literature. The hardness, transverse rupture strength, and fracture toughness of cermets prepared by2017, the10,solid Materials 1090 phase reaction and liquid phase sinter-HIP were measured as 92.3 3 of 7HRA, 2810 MPa, and 11.6 MPa·m1/2 , respectively (see more transverse rupture strength details in Figure S1 The measured properties of Ti(C,N)-based cermets are listed in Table together withphase comparisons in supplementary data). The relative densities of cermets prepared by1,direct liquid sinter-HIP, from the literature. The hardness, transverse rupture strength, and fracture toughness of cermets and by solid phase reaction and liquid phase sinter-HIP were 99.6% and 99.8%, which means that the by the solid phase reaction and liquid phase sinter-HIP were measured as 92.3 HRA, extraprepared solid phase reaction can enhance the densification process, reduce the porosity, and improve 2810 MPa, and 11.6 MPa·m1/2, respectively (see more transverse rupture strength details in Figure S1 the transverse rupture strength of cermets. Compared with the values of mechanical properties in in supplementary data). The relative densities of cermets prepared by direct liquid phase sinter-HIP, Ti(C,N)-based cermets manufactured by the sinter-HIP, vacuum sintering, and hot pressing [10,15,16], and by solid phase reaction and liquid phase sinter-HIP were 99.6% and 99.8%, which means that the the present Ti(C,N)-based cermets exhibited an excellentprocess, mechanical properties. Therefore, the extra extra solid phase reaction can enhance the densification reduce the porosity, and improve solid the phase reaction can improve mechanical properties of Ti(C,N)-based cermets, especially transverse rupture strength the of cermets. Compared with the values of mechanical properties in the transverse rupturecermets strength. Ti(C,N)-based manufactured by the sinter-HIP, vacuum sintering, and hot pressing [10,15,16], the present Ti(C,N)-based cermets exhibited an excellent mechanical properties. Therefore, the extra Tablereaction 1. Properties of Ti(C,N) prepared by solid phase reactioncermets, and sinter-HIP. solid phase can improve thecermets mechanical properties of Ti(C,N)-based especially the transverse rupture strength. Specimen
Density
Relative
Specimen Present work
Present work [10] Present [15] work [16][10] [15] [16]
solid phase Sintering Density 3) Method reaction + liquid (g/cm7.120 phase sinter-HIP solid phase liquid phase 7.120 reaction + liquid 7.105 sinter-HIP phase sinter-HIP vacuum sintering – liquid phase vacuum sintering 7.105 – sinter-HIP hot pressing – vacuum sintering -vacuum sintering hot pressing
---
Relative Density 99.8 (%)
Rockwell A Hardness 92.3
99.899.6
92.3 92.2
–
92.2
99.6 –
92.2 90.3
----
–
(MPam1/2 )
(GPa)
(%)
Present work
Fracture
Vickers
Rockwell
Density Sintering Method of Ti(C,N) Table 1. Properties by solid phaseHardness reaction andToughness sinter-HIP. A Hardness (g/cm3 ) cermets prepared
92.2 90.3 91.0
91.0
Transverse Rupture Strength (MPa)
Transverse Vickers Fracture Rupture Hardness Toughness 1680 ± 20 11.6 ± 0.3 2810 ± 80 Strength (MPa) (GPa) (MPam1/2) 1680 ± 1610 20 ± 10 11.6 ± 0.3 9.6 ± 0.2
–
2810 1820 ± 80 ± 70
9.2
1804
1610 ± 10 1380 9.6 ± 0.2 10.2
1820 ± 70 1505
-1380 1500
1500
9.2 10.2 8.3
8.3
1804 1505 1200
1200
Figure 2 shows the crack propagation of Ti(C,N)-based cermets. The indentation crack is extended along the Figure grain boundary in cermet prepared byofdirect liquid phase sinter-HIP, and the Ti(C,N) grains 2 shows the crack propagation Ti(C,N)-based cermets. The indentation crack is extended along grain boundary in cermet prepared by direct liquid phase2a. sinter-HIP, the that cannot influence thethe propagation of crack path distinctly, as shown in Figure Figure 2band shows Ti(C,N) grains deflection cannot influence the propagation crack path distinctly, as and shown in Figurechanged 2a. the mode of crack in cermet prepared byofthe solid phase reaction sinter-HIP Figure 2b shows that the mode of crack deflection in cermet prepared by the solid phase reaction and into a trans-granular fracture. This hints that the excellent bond strength of the core/rim/binder changed into a trans-granular This hintsthe thatcrack the excellent strength the grainsinter-HIP boundary in Ti(C,N)-based cermetsfracture. may influence spread bond in the crack of extension core/rim/binder grain boundary in Ti(C,N)-based cermets may influence the crack spread in the crack direction, generate an amount of resistance to the crack propagation, and enhance the toughness of extension direction, generate an amount of resistance to the crack propagation, and enhance the cermets drastically. toughness of cermets drastically.
(a)
(b) 2 μm
intergranular fracture
2 μm
transgranular fracture
5 μm
5 μm
Figure 2. SEM micrographs of crack with Ti(C,N)-based cermets prepared by direct liquid phase
Figure 2. SEM micrographs of crack with Ti(C,N)-based cermets prepared by direct liquid phase sinter-HIP (a), and solid phase reaction and liquid phase sinter-HIP (b). sinter-HIP (a), and solid phase reaction and liquid phase sinter-HIP (b).
The wear properties and compositions of abrasive dust of Ti(C,N)-based cermets with the different processes are shown in Figure 3. Figure 3aof shows that the extra solid phase The wearsintering properties and compositions of abrasive dust Ti(C,N)-based cermets with reaction the different can contribute to the reduction of the depth value of the wear track and wear volume in the process can sintering processes are shown in Figure 3. Figure 3a shows that the extra solid phase reaction of abrasion. It can be seen from Figures 3b,c that the W element contents of the abrasive dust are
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of the depth value of the wear track and wear volume in the process 4 of 7 of abrasion. It can be seen from Figure 3b,c that the W element contents of the abrasive dust4 are Materials 2017, 10, 1090 of 7 26.87 wt wt % % and and41.18 41.18wt wt% %in inthe theTi(C,N)-based Ti(C,N)-based cermets cermets with with the the extra extra solid solid phase phase reaction reaction and and direct direct 26.87 liquid phase sinter-HIP. cermets with solid phase reaction 26.87 wt % and 41.18 wt % in the Ti(C,N)-based cermets with the extra phase reaction and direct liquid phase sinter-HIP. This means that Ti(C,N)-based cermets with the extra solid phase reaction can possess higher abrasion resistance and achieve a larger quality of W element from the WC-Co liquid phasehigher sinter-HIP. Thisresistance means that Ti(C,N)-based cermets with extra solid phase can possess abrasion and achieve a larger quality ofthe W element from the reaction WC-Co ball in the the wear wear track cermets. According to Table Table and quality Figure 2, 2, the solid phase phase reaction and can in possess higher abrasion resistance and achieve a larger of the W element fromreaction the WC-Co ball track of cermets. According to 11 and Figure solid and sinter-HIP can bring about the higher transverse rupture strength and fracture toughness of cermets, ball in the wear track of cermets. According to Table 1 and Figure 2, the solid phase reaction and sinter-HIP can bring about the higher transverse rupture strength and fracture toughness of cermets, can the outstanding bond strength ofrupture theofinterface among grain boundaries and improve sinter-HIP can bring about the higher transverse strength andamong fracture toughness of cermets, which caninduce induce the outstanding bond strength the interface grain boundaries and the abrasion resistance. which can the outstanding bond strength of the interface among grain boundaries and improve theinduce abrasion resistance. improve the abrasion resistance.
Figure 3. The wear depth of cermets (a) and SEM images of the wear track for cermets with the direct Figure 3. The wear depth of cermets (a) and SEM images of the wear track for cermets with the direct liquid (b)ofand extra(a) solid (c). Figurephase 3. Thesinter-HIP wear depth cermets andphase SEM reaction images of the wear track for cermets with the direct liquid phase sinter-HIP (b) and extra solid phase reaction (c). liquid phase sinter-HIP (b) and extra solid phase reaction (c).
To investigate the morphology and structure of the cermets in depth, HRTEM of cermets were To morphology and structure depth, HRTEM of were To investigate investigate the morphology andmicrograph structure of ofofthe the cermets inand depth, HRTEM of cermets cermets were performed. Figure 4the shows the HRTEM thecermets core/rimin rim/binder grain boundaries, performed. Figure 4 shows the HRTEM micrograph of the core/rim and rim/binder grain boundaries, performed. 4 shows the HRTEM micrograph of thein core/rim andItrim/binder grain boundaries, and the FastFigure Fourier Transformation patterns are shown the insets. can be seen from Figure 4a and the Fast Fourier Transformation are shown It can can becore seenand from 4a and the thecore Fastand Fourier Transformation patterns shown in the insets. It be seen from Figure 4a that rim grain of preparedpatterns cermetsare match wellin atthe the insets. boundary. The rimFigure crystals that the core and rim grain of prepared cermets match well at the boundary. The core and rim crystals that the core and rim grain of prepared cermets match well at the boundary. The core and rim crystals have no spatial orientation difference or obvious lattice distortion. The measured interplanar have difference orrim obvious lattice distortion. The respectively, measured interplanar distances have no nospatial spatial orientation difference or distortion. The measured distances for theorientation (−11−1) core and (−11−1) are obvious 0.248 nmlattice and 0.252 nm, which interplanar reveals the for the ( − 11 − 1) and ( − 11 − 1) are 0.248 nm and 0.252 nm, respectively, which reveals the high core rim lattice distances for theof (−11−1) core and (−11−1) rim are 0.248 nmisand 0.252The nm,excellent respectively, whichofreveals the high coherence the interface (the mismatch 0.98%). matching neighbor coherence of the interface (the lattice mismatch is 0.98%). The excellent matching of neighbor crystal high coherence the interface (theboundary lattice mismatch 0.98%). The interface excellentstability matching ofimproves neighbor crystal lattices atof the core/rim grain increasesisthe core/rim and lattices atstrength the core/rim grain boundary increases core/rim interface stability and and improves the crystal lattices at the core/rim grain boundary the core/rim stability improves the bond of the grain boundary. Figureincreases 4b the shows that there isinterface a excellent match between rim bond strength of the grain boundary. Figure 4b shows that there is a excellent match between rim the bond strength boundary. Figure shows thatphase there have is a excellent match between rim and binder phasesof atthe thegrain interface. The rim grain4band binder an orientation relationship and binder phases at the interface. The rim grain and binder phase have an orientation relationship and(200) binder phases at the interface. The rim grain and binderdistances phase have orientation of rim//(11−1) binder at the interface, whose interplanar are an 0.221 nm andrelationship 0.216 nm, of //(11 −binder 1)isbinder at the interface, whose interplanar distances are 0.221 nmand andThe 0.216 nm, of (200) (200)rim rim//(11−1) at the interface, interplanar distances are nm 0.216 nm, respectively. There the misfit of 2.3% whose between rim and binder phase at0.221 the interface. lattice respectively. Therethe is the misfit 2.3% between and binder phase at the interface. The respectively. There is the misfit of ofphase 2.3%boundary between rim rim and the binder phase atand the binder interface. The lattice lattice distortion between rim/binder reduces misfit of rim crystals along distortion between the rim/binder phase boundary reduces the misfit of rim and binder crystals along distortion thebinder rim/binder phase boundary reduces the misfit of rim/binder rim and binder along the (200)rimbetween and (11−1) plane and enhances the bond strength of the graincrystals boundary. the (200) and (11 − 1) plane and enhances the bond strength of the rim/binder grain boundary. binder the (200)rim rim and (11−1)binder plane and enhances the bond strength of the rim/binder grain boundary.
Figure 4. Cont.
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Figure 4. HRTEM micrograph of the core/rim interface (a) (corresponding to region A in Figure 1) Figure 4. HRTEM micrograph of the core/rim interface (a) (corresponding to region A in Figure 1) and andrim/binder the rim/binder phase interface in prepared cermets (b) (corresponding to region in Figure the phase interface in prepared cermets (b) (corresponding to region B in B Figure 1). 1).
In the sintering process of Ti(C,N)-based cermets, Ti(C,N) particles can dissolve in the binder sintering processand of Ti(C,N)-based cermets,could Ti(C,N) particles can dissolve in the phaseInatthe high temperatures, Ti, C, and N elements reprecipitate as Ti(C,N) from the binder binder phase at high temperatures, and Ti, C, and N elements could reprecipitate as Ti(C,N) from the binder phase. The addition of carbides can precipitate as the rim phases in the form of a complicated phase. The solution. addition During of carbides can precipitate assintering the rim process, phases in of amay complicated (Ti,M)(C,N) the direct liquid phase thethe fastform heating cause an (Ti,M)(C,N) solution. During the direct liquid phase sintering process, the fast heating may cause unordered formation of the microstructure, induce the generation of core/rim/binder interfaces with an unordered formation of the microstructure, induce the generation of core/rim/binder interfaces incoherent structure, and form a Ti(C,N)/binder grain boundary with weak bond strength, which can with structure, and form a Ti(C,N)/binder grain boundarydirect with liquid weak phase bond sintering strength, causeincoherent a decrease in the toughness [10,17]. Compared to the conventional which can cause a decrease in the toughness [10,17]. Compared to the conventional direct liquid phase method for controlling the formation of core/rim structure in cermets [9,10], the solid phase reaction sintering method controlling the formation of core/rim structure cermets [9,10], the solid and liquid phasefor sinter-HIP method can solve the problem of a in lack of bond strength at phase grain reaction and liquid phase sinter-HIP method can solve the problem of a lack of bond strength atat grain boundaries. During the sintering process, the formation of the rim structure can be achieved the boundaries. During the sintering process, the formation of the rim structure can be achieved at the solid stage sintering. Mo2C and TaC can react with Ti(C,N) particles and form the rim phase during solid stage sintering. Mobefore TaC °C. canThe reactsolid withstage Ti(C,N) particles form the the diffusion rim phasereaction during 2 C and1300 the solid stage sintering sintering canand enhance ◦ C. The solid stage sintering can enhance the diffusion reaction the solid stage sintering before 1300 between WC and Ti(C,N) particles, and form the rim phase (see Figures S2 and S3 in supplementary between WC and the Ti(C,N) particles, form thethe rimcrystal phase (see Figures S2 and S3 in supplementary data). Moreover, rim phase canand grow along structure of Ti(C,N) core by an oriented data). Moreover, the rimand phase can grow along theconsistency crystal structure of Ti(C,N) core bywith an oriented attachment mechanism, achieve the structural of the core/rim interface various attachment mechanism, and achieve the structural consistency of the core/rim interface with various constituents. Moreover, in the liquid phase sintering process, the dissolution of (Ti,M)(C,N) from the constituents. Moreover, in the liquid phase sintering process, the dissolution of (Ti,M)(C,N) from the Ti(C,N)/(Ti,M)(C,N) core/rim grains can generate a (Ti,M)(C,N)-rich layer at the interface between Ti(C,N)/(Ti,M)(C,N) core/rim grains candifference generate a layerinterface at the interface between (Ti,M)(C,N) and the binder phase. The in(Ti,M)(C,N)-rich the core/rim/binder relationships of (Ti,M)(C,N) and the binder phase. The difference in the core/rim/binder interface relationships of cermets fabricated by diverse sintering methods can cause variation in the toughness. It is well known cermets fabricated by diverse sintering methods cause variation in the It is well known that the transverse rupture strength and fracturecan toughness of cermets aretoughness. strongly dependent on the that the transverse rupture strength and fracture toughness of cermets are strongly dependent the interface characteristics between the metal matrix and ceramic particles [18]. Toughening viaon crack interface characteristics between the metal matrix and ceramic particles [18]. Toughening via crack deflection can be established for brittle materials [19], and the strain energy release rate of cermets, deflection be established forthe brittle materials and the strain energy release rate ofstructure cermets, Gcermet, can can be estimated through brittle fracture[19], of Ti(C,N)/(Ti, W, Ta, Mo)(C,N) core/rim G , can be estimated through the brittle fracture of Ti(C,N)/(Ti, W, Ta, Mo)(C,N) core/rim structure cermet and the plastic rupture of (Co,Ni). The combined effects of the core/rim structure and Co/Ni binder and the plastic rupture can be described by: of (Co,Ni). The combined effects of the core/rim structure and Co/Ni binder can be described by: Gcermet (1 V − Vbinder)G )Gcore-rim core-rim + Vbinderσodbinderχ (1) Gcermet = (1= − + Vbinder σo dbinder χ (1) binder where G Gcore-rim isisthe release rate rateofofthe thecore-rim core-rimstructure, structure, bulk flow stress of where thestrain strain energy energy release σoσiso is thethe bulk flow stress of the core-rim the binder, χ is the function bond strength in the rim/binder χ canwith increase with binder, and χand is the function of bondofstrength in the rim/binder interface.interface. χ can increase increasing increasing the bond strength of the interface. The drastically reduced lattice misfit at the the bond strength of the interface. The drastically reduced lattice misfit at the core/rim/binder core/rim/binder interfaces stabilize the coherent interface lowering elastic phase interfaces phase can stabilize the can coherent interface by lowering the by elastic strain the energy [20]strain and energy [20] and improve the bond strength of the grain boundary, which can enhance the mechanical improve the bond strength of the grain boundary, which can enhance the mechanical properties and properties and abrasion resistance. Therefore, Ti(C,N)-based prepared by thereaction extra solid abrasion resistance. Therefore, Ti(C,N)-based cermets preparedcermets by the extra solid phase can phase reaction can possess a core/rim/binder grain boundary with coherent structure, which can possess a core/rim/binder grain boundary with coherent structure, which can enhance the mechanical enhance the mechanical properties and abrasion resistance of the Ti(C,N)-based cermets. properties and abrasion resistance of the Ti(C,N)-based cermets.
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4. Conclusions Ti(C,N)-based cermets with ultrahigh transverse rupture strength can be prepared by the solid phase reaction and liquid phase sinter-HIP technique. It was found that the core/rim/binder grains can have a coherent interface structure at the grain boundary, which can be favorable to enhance the mechanical properties and improve the abrasion resistance of Ti(C,N)-based cermets. The successful synthesis of the coherent phase boundary should be suitable for the manufacture of high-quality Ti(C,N)-based cermets, and provide a novel approach to achieve excellent properties in cermets. Supplementary Materials: The following are available online at www.mdpi.com/1996-1944/10/9/1090/s1, Figure S1: The curve of transverse rupture strength for cermets prepared by the extra solid phase, Figure S2: SEM micrographs of microstructure in Ti(C,N)-based cermets sintered at 1300 ◦ C for 0 h (a) and 2 h (b), Figure S3: XRD patterns of Ti(C,N)-based cermets sintered at 1300 ◦ C for 0 h and 2 h. Acknowledgments: This work is supported by Research Funds for Central Universities (531107040967) and National Natural Science Foundation of China (No. 51634006). Author Contributions: Nan Lin and Yuehui He conceived of and designed the experiments. Nan Lin performed the experiments. Nan Lin and Xiyue Kang analyzed the data. Nan Lin and Yuehui He wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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