Microstructure and cutting performance investigation of Ti(C, N)-based cermets containing various types of secondary carbides

In this paper, the effects of type and amount of various secondary carbides on the microstructure and cutting performance of Ti(C_0.7N_0.3)–12Co system have been investigated. Nine alloys with different amounts of WC, Mo₂C and Ta_0.7Nb_0.3C additives were studied, using scanning electron microscopy (SEM), Vickers hardness (HV), transverse rupture strength (TRS) and continuous turning tests. Alloys including Mo₂C and WC presented finer carbonitride grain structure, as well as higher wear resistance resulting from their higher hardness and highly solution hardened binder phase.     

Mo element-induced wear behavior evolution of self-lubricant Ti(C,N)-based cermets

In the work, Mo element is used to adjust composition and distribution of the lubricating phase of the self-lubricant Ti(C, N)-based cermets. Ring-block wear test of self-mate pairs is conducted. The microstructure and wear behavior evolution of the cermets are investigated. The results indicate by adjusting the content of Mo, the distribution of graphite on the surface of the self-lubricant cermets changes. With the increase of Mo content, the volume of graphite decrease, and the undissolved Mo increase. During self-mate pair sliding wear, the tribolayer transform from graphite to graphite+MoOx and then to MoOx. The cermets with 4 wt%. Mo show the best tribological properties, which shows a smooth and dense composite tribolayer formed mainly by graphite and MoOx.     

Ti (C,N)-based cermets sintered under high pressure

In this study, high pressure and high temperature sintering (HPHT) is adopted in the cermet fabrication process, and the microstructure and mechanical properties of cermets with TiC_0.5N_0.5–15WC–10Mo₂C–5TaC–10Ni–10Co (wt%) sintered under 5 GPa and different temperatures (900–1600 °C) using 6 × 14 MN cubic press are investigated. Results show that the densities of samples can reach up to 7.00 g/cm³. Vickers hardness and fracture toughness of the products are over 1727 HV30 and 7.2 MPa m^1/2 respectively. In addition, the sintering results are compared with the data that obtained from commercial samples which produced via conventional sintering technique. The conclusion is that high density and high hardness cermets can be obtained through HPHT sintering.     

粘结剂成分对Ti(C,N)基金属陶瓷腐蚀行为的影响

研究了含38 wt% Ni或Ni–20Cr粘结剂的2种Ti(C,N)基金属陶瓷在0.2 M H2SO4和0.2 M NaOH溶液中的室温腐蚀行为。在0.2 MH2SO4溶液中,2种金属陶瓷的腐蚀行为和耐蚀性存在一些明显的差异：与以Ni作粘结剂的金属陶瓷不同,以Ni–20Cr作粘结剂的金属陶瓷浸泡过程中表面自发生成NiO、Ni(OH)x(SO4)y、Cr2O3和Cr(OH)3,致使粘结相溶解十分缓慢;动电位极化过程中不发生钝化,伪钝化后电流密度增加较快。在0.2 M NaOH溶液中,2种金属陶瓷的腐蚀行为和耐蚀性无明显差异：浸泡过程中陶瓷晶粒溶解十分缓慢,同时生成NiOOH和Cr6+化合物;动电位极化过程中不发生钝化,但发生伪钝化。     

Preparation of self-lubricating Ti(C,N)-based cermets by solid carburization and wear behavior

In this work, self-lubricating Ti(C,N)-based cermets were prepared by solid carburization. The sintered cermets were wrapped by carburizing agent and sintered again at 1440 °C with different time. The microstructure and composition of cermets were studied. The wear behavior of cermets containing graphite phase was also evaluated using a block-on-ring tribometer. The results showed that the carbon content increased gradually in binder phase with carburizing time. When the carburizing time was 3 h, the carbon got saturated in binder phase. When the carbon content exceeded the solubility in the binder, excessive carbon precipitated and formed graphite phase. Uniformly distributed graphite clusters formed in cermets after carburization for 4 h. The graphite clusters consisted of flocked graphite particles. With the carburizing time extended to 5 h, the graphite clusters became large and some of them interlaced together. Besides, the wear results indicated that the volume loss of cermets containing graphite phase was half of that without graphite due to the formation of smooth tribofilm on the worn surface of cermets.     

Effect of secondary carbides addition on the microstructure and mechanical properties of (Ti, W, Mo, V)(C, N)-based cermets

(Ti, W, Mo, V)(C, N)-based cermets were prepared by mixing Mo₂C, WC and TaC with ultrafine (Ti, W, Mo, V)(C, N) powders, and then processed via a conventional P/M technique. The effect of Mo₂C, WC and TaC on the microstructure and mechanical properties of (Ti, W, Mo, V)(C, N)-8 wt.% Ni-7 wt.% Co systems was investigated. The Mo₂C content was varied from 0 to 10 wt.% and additive WC or TaC was added at a level of 5 wt.% with Mo₂C addition. The results show that the densification of (Ti, W, Mo, V)(C, N)-8 wt.% Ni-7 wt.% Co cermets was improved significantly by the addition of Mo₂C. With the increase of Mo₂C content, there is a coarsening tendency in the microstructure of (Ti, 20W, 15Mo, 0.2V)(C, N)-8Ni-7Co system, but the refinement for (Ti, 15W, 5Mo, 0.2V)(C, N)-8Ni-7Co. TaC addition decreases the density of (Ti, 15W, 5Mo, 0.2V)(C, N)-10Mo₂C-8Ni-7Co cermet and thus weakens its bending strength. (Ti, 15W, 5Mo, 0.2V)(C, N)-10Mo₂C-5WC-8Ni-7Co cermet has optimal mechanical properties: bending strength of 1999 MPa, hardness (H_v) of 1677 MPa and toughness of 9.95 MPa m^1/2 respectively by adding WC, which is due to its ultrafine and weak core/rim structure.     

纳米TiN改性Ti（C, N）基金属陶瓷的组织和性能

采用真空烧结法制备纳米TiN改性的Ti（C, N）基金属陶瓷,研究不同金属相对纳米改性Ti（C, N）基金属陶瓷组织和力学性能的影响。结果表明：除经典的黑芯/灰壳组织外,添加纳米TiN的金属陶瓷在黑芯内还出现灰芯结构;纳米TiN主要分布在陶瓷相颗粒的晶界处;相对于未添加纳米TiN的金属陶瓷,添加纳米TiN粉末能明显提高金属陶瓷的抗弯强度、硬度与断裂韧性;对纳米TiN改性的金属陶瓷而言,金属相Ni能提供更好的抗弯强度与断裂韧性,而金属相Co则能带来更高的硬度。     

纳米改性Ti(C,N)基金属陶瓷的抗热震性能研究

为研究不同金属相对纳米TiN改性Ti(C,N)基金属陶瓷抗热震性能的影响,采用真空烧结法分别制备了Ti(C,N)基20Ni、10Co-10Ni和20Co三组金属陶瓷,用急冷-强度法测定了其抗热震性能.结果表明,3组金属陶瓷试样的热震残留强度均随热震温度的升高而降低,其热震残留强度急剧下降的临界温度分别为410、370和330℃.扫描电镜分析结果表明,热震后试样的组织仍然保持着金属陶瓷典型的芯/壳结构,随着热震温度的升高,组织中出现裂纹,断口中有气孔及裂纹出现,导致材料强度下降.对热震试样硬度的测试结果表明,纳米TiN改性Ti(C,N)基金属陶瓷在热震后仍然具有很高的硬度.     

Effects of Cr3C2 addition on the corrosion behavior of Ti(C,N)-based cermets

The Ti(C, N)-based cermets with different Cr₃C₂ addition were prepared and the effects of Cr₃C₂ addition on the microstructure and properties of cermets were discussed. The corrosion behavior of the cermets with different Cr₃C₂ addition was investigated emphatically in 2 mol/L HNO₃ solution. The results indicate that there is no obvious effect of Cr₃C₂ addition on the densification of the cermets, and all cermets are almost fully densified during sintering. The thickness of rim phase is reduced and the core size is increased remarkably in the cermets with 1 wt.% and 3 wt.% Cr₃C₂ addition; the grains are refined significantly in the cermets with the increase of Cr₃C₂ addition to 5 wt.%. The hardness and transverse rapture strength of the cermets are improved with Cr₃C₂ added properly. In HNO₃ solution, the corrosion resistance of cermets is improved remarkably by Cr₃C₂ addition. The corrosion of binder phase is predominant in the cermets in which the Ni binder phase without Cr has lower corrosion resistance than the rim phase; whereas the corrosion resistance of binder phase with high Cr content is better compared to the rim phase, so that the degradation of rim phase is predominant and a reticulate binder phase forms. With the increase of Cr₃C₂ addition, the Mo content in rim increases, and it is bad for the corrosion resistance of rim phase. Additionally, the inner rim phase has lower corrosion resistance than the outer rim phase owing to the higher Mo content.     

Ti(C,N)基金属陶瓷断口形貌及增韧机理

采用真空烧结工艺制备Ti(C,N)基金属陶瓷,测定了材料的力学性能。结果表明,其力学性能与未加纳米粉的金属陶瓷相比,硬度略有升高,但横向断裂韧性提高了近一倍。断口形貌和微观组织分析表明:金属陶瓷的断口形貌与其强韧性有着密切的关系。纳米粉的加入降低了原始粉末的平均粒度,使得金属陶瓷硬质相的粒度降低,减小了晶粒间的平均自由程。镶嵌于大颗粒环形相和弥散分布于粘结相中的细小硬质相颗粒,对裂纹的形成和扩展起到阻碍作用,会使金属陶瓷因裂纹扩展途径发生偏转而增韧。     

Early high-temperature oxidation behavior of Ti(C,N)-based cermets with multi-component AlCoCrFeNi high-entropy alloy binder

The early high-temperature oxidation behavior of Ti(C,N)-based cermets with equiatomic AlCoCrFeNi high-entropy alloy binder has been studied, as well as the cermet with Ni/Co binder as a reference. Experiments were performed at the temperature range of 800–1100 °C in static laboratory air. The micro-structural evolution of the multi-layered oxide scales on the cermets was investigated and the effect of binder phase constituent on the oxidation characteristics of Ti(C,N)-based cermets was evaluated. The results demonstrated that the cermet with AlCoCrFeNi multi-element alloy binder possesses superior oxidation resistance, which is greatly better than that of the cermet with Ni/Co metallic binder under the same condition. We suggest that the formation of a continuous and dense external oxide scale can effectively impede the outward diffusion of volatile tungsten oxides and inward oxygen transport, leading to a remarkable improvement of oxidation resistance. In addition, the enhanced oxidation resistance was related to the high Cr and Al concentration in AlCoCrFeNi binder phase that urges the formation of oxide layers with more efficient passivation effect against oxidation.     

Effect of submicron Ti(C,N) on the microstructure and the mechanical properties of Ti(C,N)-based cermets

There is an increased industry demand for Ti(C,N)-based cermets with improved material properties. One of the parameters which are supposed to influence these properties is the mean particle size of the Ti(C,N) powder used. In this study the effects of a newly developed submicron Ti(C,N) powder grade on the properties of Ti(C,N)-based cermets, including hardness, toughness and microstructure were investigated. The cermets showed only small differences with respect to outgassing upon sintering (investigated by MS-EGA) as well as shrinkage (dilatometry). Cermet formulations with submicron Ti(C,N) could be sintered under identical conditions as with fine Ti(C,N), yielding completely dense bodies of A00 porosity. From SEM and XRD investigations it was found that submicron Ti(C,N) powders cause accelerated diffusion and homogenisation of the microstructure leading to a substantially increased amount of outer rim phase, a higher amount of inverse grains and substantially finer and less Ti(C,N) cores. Upon using submicron Ti(C,N), hardness (HV10) is increased and in one grade the fracture toughness (Palmqvist–Shetty) is increased as well.     

Tribological performances of Ti(C,N)-based cermets with different graphite contents in dry sliding condition

Ti(C,N)-based cermets with different graphite (Gr) contents were prepared to solve the problem that the cermets self-mated couples suffer severe wear in dry sliding condition. The microstructure and mechanical properties of cermets with varying graphite contents were observed and evaluated, and the effects of graphite content on the tribological performances of the cermets self-mated rubbing pairs were investigated using a block-on-ring apparatus at room temperature. The results show that the distribution of graphite phase in cermets transforms from segregated floc-like to uniform granular with the increase of graphite content. The hardness and fracture toughness of the cermets self-lubricating materials rise firstly and then fall, whereas the transverse rupture strength decreases gradually with graphite adding. Compared with the cermet without graphite, the tribological properties of the cermets containing graphite are improved significantly. When the graphite addition reaches 0.8 wt%, the cermet exhibits the optimal wear resistance, which is attributed to the formation of a stable lubricating film composed of graphite and metal oxides (TiO₂, MoO₃, NiO) on the tribo-surface. Additionally, results reveal that the wear mechanism of cermets in dry condition is a complicated regime containing abrasion, adhesion and oxidation. After adding graphite, the adhesive wear of cermets is inhibited effectively, however the abrasive wear will be aggravated as the graphite content increases continuously.     

Ultrafine (Ti, M)(C, N)-based cermets with optimal mechanical properties

A tough and strong cermet with the composition (Ti,20M,0.2V)(C,N)-16M-20Ni/Co (M = W,Mo) was prepared by mixing WC and Mo₂C with ultrafine (Ti,M)(C,N) powders, and then, processed via a conventional P/M technique. It has an ultrafine and distinct core/rim structure, resulting in excellent mechanical properties: bending strength of 2210 MPa, HV hardness of 14.7 GPa and toughness of 10.1 MPa m^1/2. The small concentration gradient in core/rim composition and the disappearance of inner rims benefit the reduction of the stress concentration at the core/rim interface in (Ti,M)(C,N)-M_xC cermets, and thus improve their toughness. In addition, ultrafine microstrucure improves mainly their bending strength and hardness.     

Effect of graphite size on the tribological behavior of Ti (C,N)-based cermets self-mated wear pairs

In the present work, graphite of different particle size ranging from 2 μm to 32 μm is added into the system to lubricate the Ti (C, N) cermets, but the addition amount is fixed at 1 wt%. The sliding wear tests were carried out using block-on-ring equipment. The distribution morphology of graphite, mechanical properties and wear behavior were studied in this paper. The results indicate that when the graphite is added into the cermet, graphite particles join together and are distributed to clusters of different size which are like bird nests. The range of the “bird-nest” clusters increase with the improvement of graphite size when the graphite particle size is smaller than 12 μm but decrease when larger than 12 μm. Furthermore when the graphite grain size increase to 32 μm, most of the particles exist in the cermets individually instead of distributing like bird nest The mechanical properties decrease with the improvement of graphite size, and when the particles size of graphite increase to 32 μm the mechanical properties get worse sharply. The wear mechanism of the cermets with graphite, adhesion and plastic deformation are dominated. The extruded graphite forms a tribofilm, and protects the surface from the hard abrasive particles ploughing. When the particles size of graphite added is 22 μm, cermets which have coarser graphite particles and more but moderate-in-size graphite clusters possess the best effect of lubrication     

Early high temperature oxidation behaviour of Ti(C,N)-based cermets in air

Isothermal oxidation behaviour of two Ti(C,N)-based cermets (TiC-10TiN-16Mo-6.5WC-0.8C-0.6Cr₃C₂-(32-x)Ni-xCr, x = 0 and 6.4 wt%) was investigated in air at 800-1100 °C up to 2 h. Mass gains exhibited neither linear nor parabolic law during isothermal oxidation. The oxide scales formed at 800-1100 °C were multi-layered, consisting of NiO outerlayer, NiTiO₃ interlayer and TiO₂-based innerlayer. The internal oxidation zones formed at 1000-1100 °C consisted of Ti-based, Ni-based and Mo-based complex oxides. Cermet with 6.4 wt% Cr exhibited superior oxidation resistance, due to the presences of Cr_0.17Mo_0.83O₂ in TiO₂-based innerlayer of the oxide scale and Cr-rich Ti-based complex oxide in the internal oxidation zone.     

Microstructure and cutting performance of Ti（C, N）-based cermets heat-treated in nitrogen

Ti（C, N）-based cermets were treated using hot isostatic pressing （HIP） at 1423 K in nitrogen. The microstructures compared with the as-sintered cermets were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray analysis, and electron microprobe analysis. It was found that high nitrogen activity in the surface zone resulted in the formation of gradient structure. Approximately 20-1am-deep, nitrogen-rich and titanium-rich hard surface zone was introduced by the heat treatment. The nitrogen activity was the driving force that caused the transportation of the atoms through the binder, titanium towards the surface, and tungsten and molybdenum inwards. In the surface zone, the particle size became fine, the inner rim disappeared, and the volume fraction of the outer rim and the binder phase considerably reduced. Small grains of TiN, WC, Mo2C, and nitrogen-rich carbonitlide phases formed in the surface zone during the heat treatment, improving the tlibological property of the heat-treated cermet.     

Influence of preparation process on sintering behavior and mechanical properties of ultrafine grained Ti（C, N）-based cermets

The influences of forming and sintering processes on distortion, cracking as well as mechanical properties of sintered bodies of ultrafine grained Ti（C, N）-based cermets were investigated. The results show that lubricant is indispensable to fabrication of ultrafine Ti（C, N）-based cermets, however, with low binder content in powder mixture, the lubrication action of paraffin is attenuated. A appropriate level of 2% （mass fraction） paraffin is determined for a cermet with binder content of 36% （mass fraction）. It is also found that the influence of compaction pressure on distortion and cracking of sintered bodies presents a complex relationship. A relatively lower or higher compaction pressure, less than 100 MPa and more than 400 MPa respectively, favors uniform density distribution in green compact. The heating rate of sintering should be strictly controlled. Too fast heating rate results in enclosed pores to burst and forms large size pores in sintering body. A heating rate of 3 ℃/min is recommended.     

Effect of sintering on microstructures and properties of sub-micron Ti （C, N）-based cermets

The influence of different sintering processes, including vacuum sintering, vacuum sintering followed by HIP and sintering-HIP, on the microstructure and properties of sub-micron Ti(C,N) cermets with various binder contents was studied. Image analysis based on back-scattered electrons image observations was used to determine the morphologic and structural characteristics. Transverse rupture strength(TRS), hardness, fracture toughness were measured and TRS data were treated by Weibull statistics further. It is shown that a very significant improvement in TRS can be obtained by HIP or sintering-HIP treatment for the alloys with lower and middle binder content at the controlled cooling rate, but the effect is not obvious for the alloys with higher binder content. HIP is also helpful for improving the hardness of sub-micron Ti(C,N) cermets, however, but can lower the fracture toughness. The varia-tion of these properties was interpreted in terms of the difference in morphologic and structural characteristics.     

纳米Ti(C,N)增强Ti(C,N)基金属陶瓷的研究

采用纳米Ti(C，N)粉末制备Ti(C，N)基金属陶瓷，研究了纳米粉末对金属陶瓷组织及性能的影响。结果表明，粉末冶金过程中，纳米Ti(C，N)粉末易于在粘结相中扩散与溶解及沿晶界分布，降低了硬质相在粘结相中的溶解度，抑制了晶粒长大，提高了材料的红硬性能。抗弯强度与晶粒尺寸满足于Hall-Perch公式，5wt％～l0wt％的纳米粉末加入量可使金属陶瓷的抗弯强度和切削性能得到较大的提高，但硬度变化不大。切削磨损主要表现为磨粒磨损和轻微的粘着磨损，磨痕细小均匀。