Hardness can be defined microscopically as the combined resistance of chemical bonds in a material to indentation. The current review presents three most popular microscopic models based on distinct scaling schemes of this resistance, namely the bond resistance, bond strength, and electronegativity models, with key points during employing these microscopic models addressed. These models can be used to estimate the hardness of known crystals. More importantly, hardness prediction based on the designed crystal structures becomes feasible with these models. Consequently, a straightforward and powerful criterion for novel superhard materials is provided. The current focuses of research on potential superhard materials are also discussed. (C) 2012 Elsevier Ltd. All rights reserved.
Nanocrystalline WC-Co materials have been the subject of interests and focus of research programs around the world for the past two decades owing to the expectations that the mechanical behavior of the material may improve significantly when grain sizes reduce to nanometer scale. However, although numerous technologies are available for making nanosized tungsten carbide powders, obtaining true nanocrystalline WC-Co (average WC grain size <100 nm) has been a great challenge due to the difficulties of controlling grain growth during sintering. Evaluation of the mechanical properties of nanocrystalline WC-Co materials is also difficult because there is little published data that are based on specimens with truly nanoscale grain sizes. In this review, the challenges and results of sintering nanocrystalline WC-Co powders will be examined as well as the various technologies for producing nanosized tungsten carbide powders. It will be discussed that the key challenge to the production of bulk nanocrystalline cemented tungsten carbide materials is to control the rapid grain growth during the early stage of sintering. The current understanding on the mechanical properties of cemented tungsten carbide made from nanoscaled WC-Co powders will also reviewed. (C) 2008 Elsevier Ltd. All rights reserved.
In order to improve the performances of TiCN-based cermets, researchers have paid much attention directly towards developing various new spices of cermets. The present review will try to sum up the efforts in designing and tailing in compositions and microstructures of TiCN-based cermets in recent years aiming at enhanced cermet properties. The relationship between the cermet constituents and their mechanical properties and wear resistance, as well as the advances in the synthesis of TiCN powders and preparation of TiCN-based cermets were included. Special emphasis was paid on the preparation of ultrafine/nano TiCN-based cermets possessing enhanced hardness, mechanical strength, toughness and wear resistance, which has led to a very recent surge of interest in the development of TiCN-based cermets. In particular, it has been possible to obtain dense TiCN-based cermets with ultrafine- and/or nano-structures by means of fast sintering techniques, such as spark plasma sintering, microwave vacuum sintering and so on. (C) 2012 Elsevier Ltd. All rights reserved.
Carbon-based thin films possess unique and adjustable combination of properties such as high hardness and wear resistance, chemical resistance and good tribological performances. Among critical variables to tailor a-C film's properties for specific application is the distribution of the carbon hybridization states (sp(1), sp(2) and sp(3) bonds), the atomic H content, the content in dopants such as Si, F, N, B and O. Here we focus on: (i) a-C and hydrogenated amorphous carbon (a-C:H) films with a mixture of sp(2) and sp(3) bonding, highly sp(3)-boned material (ta-C) and sp(2)-bonded carbon, (ii) carbon nitride (CNx) coatings and (iii) metal/amorphous carbon (a-C:M) composite films. The study is focused on the review of the nanomechanical properties and analysis of the nanoscratching processes at low loads to obtain quantitative analysis, the comparison of their elastic/plastic deformation response, and nanotribological behavior of the a-C, ta-C, a-C:H, CNx, and a-C:M films. For ta-C and a-C:M films new data are presented and discussed. (C) 2009 Elsevier Ltd. All rights reserved.
Combustion synthesis is widely used for preparing various refractory and hard materials, including alloys, intermetallics, ceramics, and cermets. The unique reaction condition in combustion synthesis with extremely-high temperature and fast heating/cooling rate offers the products interesting microstructures and superior mechanical properties. In comparison with conventional powder metallurgy approaches, combustion synthesis exhibits the advantages of short processing time, less energy consumption, and lower cost, thus providing a more efficient way to produce refractory and hard materials. This article reviews recent progress in combustion synthesis of refractory and hard materials, with an emphasis on the results reported in the last decade. Both the synthesis of powders and direct fabrication of bulk materials are discussed. For the synthesis of powders, results in two aspects are reviewed, viz, synthesis of ultrafine and especially nano-sized powders by thermal reduction reactions or post chemical etching, and synthesis of nitride and carbide powders in air. For direct fabrication of bulk materials, two techniques are involved, viz, combustion synthesis with simultaneous densification assisted by a mechanical or gas pressure, and combustion synthesis casting in a high-pressure Ar atmosphere or in a high-gravity field. (C) 2012 Elsevier Ltd. All rights reserved.
The present work concerns an experimental study of hard turning with CBN tool of AISI 52100 bearing steel, hardened at 64 HRC. The main objectives are firstly focused on delimiting the hard turning domain and investigating tool wear and forces behaviour evolution versus variations of workpiece hardness and cutting speed. Secondly, the relationship between cutting parameters (cutting speed, feed rate and depth of cut) and machining output variables (surface roughness, cutting forces) through the response surface methodology (RSM) are analysed and modeled. The combined effects of the cutting parameters on machining output variables are investigated while employing the analysis of variance (ANOVA). The quadratic model of RSM associated with response optimization technique and composite desirability was used to find optimum values of machining parameters with respect to objectives (surface roughness and cutting force values). Results show how much surface roughness is mainly influenced by feed rate and cutting speed. Also, it is underlined that the thrust force is the highest of cutting force components, and it is highly sensitive to workpiece hardness, negative rake angle and tool wear evolution. Finally, the depth of cut exhibits maximum influence on cutting forces as compared to the feed rate and cutting speed. (C) 2009 Elsevier Ltd. All rights reserved.
Surface texturing with different geometrical characteristics was made on the rake face of the WC/Co carbide tools, molybdenum disulfide (MoS2) solid lubricants were filled into the textured rake-face. Dry cutting tests were carried out with these rake-face textured tools and a conventional tool. The effect of the texture shape on the cutting performance of these rake-face textured tools was investigated. Results show that the cutting forces, cutting temperature, and the friction coefficient at the tool-chip interface of the rake-face textured tools were significantly reduced compared with that of the conventional one. The rake-face textured tool with elliptical grooves on its rake face had the most improved cutting performance. Two mechanisms responsible were found, the first one is explained as the formation of a lubricating film with low shear strength at the tool-chip interface, which was released from the texturing and smeared on the rake face, and served as lubricating additive during dry cutting processes; the other one was explained by the reduced contact length at the tool-chip interface of the rake-face textured tools, which contributes to the decrease of the direct contact area between the chip and rake face. (C) 2011 Elsevier Ltd. All rights reserved.
We investigated the microstructure and mechanical properties of Ti20Zr20Hf20Nb20X20 (X = V or Cr) high-entropy alloys (HEA), produced by induction melting and casting in inert atmosphere. The structures of these alloys were studied via X-ray diffractometry and scanning electron microscopy. Results show that Ti20Zr20Hf20Nb20X20 has mainly the body centered cubic (BCC) structure, whereas the BCC matrix of Ti20Zr20Hf20Nb20X20 contains small amount of Cr2Nb and Cr2Hf intermetallic compounds. Ti20Zr20Hf20Nb20X20 alloy shows the high strength and the homogeneous deformation under compression at room temperature. The strength and hardness of Ti20Zr20Hf20Nb20X20 alloy are further enhanced by the Cr-containing Laves phases segregated during casting. The structural and mechanical properties remained almost unchanged after a short time (10 min) heat treatment at 573, 773, 973 and 1173 K indicating the resistance to working temperature peaks for these two alloys. Ab initio calculations predict ductile behavior for these and similar refractory HEM. The theoretically calculated Young's modulus E is in good agreement with the experimental ones. (C) 2014 Elsevier Ltd. All rights reserved.
Four nitride coatings, TiN, TiAlN, AlTiN and CrAlN were deposited on YG6 (WC + 6 wt.% Co) cemented carbide by cathodic arc-evaporation technique. The friction and wear properties were investigated and compared using ball-on-disc method at high speed with SiC ball as a counter material. The tests were evaluated by scanning electron microscopy, X-ray diffractometer, energy dispersive X-ray, micro hardness tester and an optical profilometer. The results showed that TiN and TiAlN coatings presented lower friction coefficient and lower wear rate, and that high Al content AlTiN and CrAlN coatings didn't present better anti-wear properties in this test Oxidation and abrasive wear were the main wear mechanism of TiN coating. In spite of the observation of micro-grooves and partial fractures. TiAlN possessed perfect tribological properties compared with the other coatings. High Al content increased the chemical reactivity and aroused severe adhesive wear of AlTiN coating. CrAlN coating presented better properties of anti-spalling and anti-adhesion, but abundant accumulated debris accelerated wear of the coating under this enclosed wear environment. (C) 2011 Elsevier Ltd. All rights reserved.
ZrB2 ceramic (Z), ZrB2-25 vol% SiC composite (ZS) and ZrB2-25 vol% SiC composite doped with 5 wt% graphite (ZSG) were fabricated by spark plasma sintering process. The sintered samples were compared to investigate the effects of submicron SiC particles and graphite nano-flakes on sinterability, microstructure and mechanical properties of ZrB2-based ceramic matrix composites. Spark plasma sintering at 1900 degrees C for 7 min under 40 MPa resulted in fully dense ZS and ZSG samples but a relative density of 96.1% was achieved for Z sample. The growth of ZrB2 grains was effectively decelerated by addition of submicron SiC particles and graphite nano-flakes. Hardness values of 13.1, 19.5 and 16.6 GPa were measured for Z, ZS and ZSG samples, respectively, which verify the hardening effect of SiC and softening effect of graphite in ZrB2 -based composites. By the simultaneous addition of SiC and graphite into ZrB2 matrix, the indentation fracture toughness of ZSG sample reached 6.7 MPa m(1/2), meaningfully higher than those of Z and ZS samples with toughness values of 3.2 and 4.3 MPa m(1/2), respectively. Such an improvement in the fracture toughness of ZSG nanocomposite was attributed to the presence of graphite nano-flakes as well as the in-situ formed ZrC and B4C nano-particles. Flexural strength of Z, ZS and ZSG samples reached 445, 624 and 631 MPa, respectively. Although SiC had a remarkable strengthening effect in the ZrB2-based ceramic, the addition of graphite together with SiC had not a significant impact on the flexural strength of composite material.
One of the main topics of the actual research in the field of cemented carbides concerns the development of new composites, with partial or total substitution of the traditional cobalt binder by other more economic and less toxic materials. Composites with partial substitution of cobalt by nickel and iron are currently entering in industrial production. However, the total cobalt replacement is envisaged and Ni-Fe or Ni-Fe-Cr alloys are being currently investigated for such a purpose. The actual knowledge on phase diagrams for WC and different binders will be extremely useful and opportune regarding the need to choose initial compositions leading to a desired final phase composition and to select adequate sintering cycle conditions. In the present review, the existent phase diagrams of W-C-M with M = (Co, Fe, Ni, Fe-Ni, Fe-Al, Co-Fe-Ni, Cr and Cr-Fe) are presented and discussed. (C) 2011 Elsevier Ltd. All rights reserved.
Thermally sprayed hardmetal coatings have a typical thickness within the range 100-500 mu m. Thus, thermal spray enables the functionality of hardmetals to be realized on the surface of large parts, which cannot be produced by powder metallurgy for technical and economical reasons. This article reviews the different types of thermal spray processes, with particular focus on the high velocity HVOF and HVAF deposition techniques which are of most relevance to the application of hardmetal coatings. Feedstock powder preparation technologies are presented. The majority of hardmetal thermal spray coatings are based either on WC or Cr3C2 or hard phases appearing as a result of their interaction. As an alternative, TiC-based compositions are most intensively studied. Thermal spraying generates significant changes in the hardmetal chemical and phase compositions between the feedstock powder to the sprayed coating. Coating formation and microstructures as well as selected properties, such as hardness, the effect of heat treatments and the oxidation in service, as well as corrosion resistance are discussed. As an example for wear protection applications, abrasion wear resistance is shortly discussed. This paper is a partly updated and condensed version of the chapter: "Coatings by thermal spray" in the book "Comprehensive Hard Materials", V.K. Sarin (Editor-in-Chief) & D. Mari & L Llanes (Vol. Ed.), Vol. 1 (pp. 471-506), Elsevier, 2014. (C) 2014 Elsevier Ltd. All rights reserved.
The intention of this paper is to give an overview of selected R&D trends in the cemented carbide field, focusing on work performed in recent years. Due to the large activity in the field, it is not feasible to give a comprehensive review of all research activities in the hard metal industry and academia. Therefore, focus has been given to areas with a large number of publications in journals on the field of cemented carbides, cermets and powder metallurgy of hard materials, which indicates interesting emerging areas, techniques and trends. Such areas include fine grained materials, interfaces, alternative binders, alternative sintering techniques, and gradients; high resolution microscopy and electron backscatter diffraction. Amongst emerging trends, coupling between experiments and modelling at different scales is growing, as well as three dimensional modelling of microstructure evolution. Trends are discussed and an outlook for future development in the respective fields is given. (C) 2014 Elsevier Ltd. All rights reserved.
Thermal spray cermet based on tungsten carbide has been widely used due to its excellent wear resistance. The features of both carbide and binder phases are essential factors which determine the performance of cermet coating. The thermal cycling of WC-Co spray particles up to a temperature over the melting point of binder phase during thermal spraying involves the decarburization of carbide. The decarburization of carbide becomes severe with the decrease of carbide particle size, which makes it difficult yet to deposit a dense nanostructured WC-Co with a limited decarburization by thermal spraying. The decarburization not only reduces the wear-resistant phase but also leads to the formation of brittle Co-W-C ternary binder phase. Moreover, the limited decarburization involves the deposition of spray particle at a solid-liquid two-phase state with carbides at a solid state and metal binder in a molten state during spraying. High velocity impact of two-phase droplets as in high velocity oxy-fuel spraying (HVOF) results in the formation of a dense cermet coating and on the other hand leads Lathe possibility of rebounding of wear-resistant solid carbide particles. In this review article, the microstructural features of thermal spray WC-Co are examined based on the effect of the decarburization of tungsten carbide. The decarburization mechanisms of tungsten carbide are discussed for the control of decarburization of carbide. The effects of carbide particle size on the deposition process, adhesion of HVOF coating and wear performance of WC-Co coating as well are examined based on a solid-liquid two-phase deposition process. It is demonstrated that WC-Co cermet with different sizes of WC particles should be deposited by different processes. Moreover, the deposition of nanostructured WC-Co by thermal spraying and recent advances on the cold spraying of nanostructured WC-Co are introduced. The cold spraying with the proper design of spray powders will become promising process to deposit nanostructured WC-Co with pure cobalt binder with the hardness comparable to a sintered bulk and even high toughness of 18.9 MPa m(1/2). The pure metastable metal binder phase evolved in the deposit makes it possible to deposit hard cermet through healing the non-bonded interfaces in the coating by post-spray annealing. (C) 2012 Elsevier Ltd. All rights reserved.
Using vacuum-arc evaporation method we fabricated periodic multilayered TiN/MoN structures with different bilayer periods X ranging from 25 to 100 nm. Rutherford backscattering (RBS), X-ray diffraction (XRD), scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) and microhardness measurements were used for investigations of composition, structure and mechanical properties of the multilayered coatings. We found that molybdenum nitride and titanium nitride layers grown on steel show local partial epitaxy and columnar growth across interfaces. A molybdenum-titanium carbide interlayer was evidenced between the substrate and the multilayer. Molybdenum nitride and titanium nitride layers contain small (5-30 nm) grains and are well crystallized with (100) preferred orientation. They were identified as stoichiometric fcc TiN and cubic gamma-Mo2N. Non-cubic molybdenum nitride phases were also detected. The hardness of the obtained structures achieved great values and maximal hardness was 29-31 GPa for multilayered structure with 50 nm period. Hardness of the obtained coatings is 25% higher in comparison with initial single-layer nitride coatings, wherein plasticity index (H/E) of multilayered structure is 0.075. (C) 2014 Elsevier Ltd. All rights reserved.
Nanoscale and microscale textures with different geometrical characteristics were fabricated on the surface of the Al2O3/TiC ceramic tool, and molybdenum disulfide (MoS2) solid lubricants were burnished into the textures. The effect of the textures on the cutting performance was investigated using the textured self-lubricated tools and conventional tools in dry cutting tests. The tool wear, cutting force, cutting temperature, friction coefficient, surface roughness and chip topography were measured. Results show that the cutting force, cutting temperature, friction coefficient and tool wear of nanoscale and microscale textured self-lubricated tools are significantly reduced compared with the conventional tool, and the developed tool with wavy microscale textures on the rake face is the most effective in improving the cutting performance. The textured self-lubricated tools increase the surface roughness of machined workpiece, while they can reduce the vibration for a stable cutting and produce more uniform surface quality. The chip topography is changed by the textured self-lubricated tools. As a result, the nanoscale and microscale textured self-lubricated tools effectively improve the cutting performance of conventional Al2O3/TiC ceramic tool, and they are applicable to a stable dry cutting of the hardened steel. (C) 2013 Elsevier Ltd. All rights reserved.
The present work is focused on investigating the effect of SiC addition (0, 15, 20, 25 and 30 vol.%) on microstructural features, phase evolution and mechanical properties of the vacuum hot pressed TiB2 at 1850 degrees C/2 h/20 MPa. Hardness measurements revealed an initial increase in hardness from 14.2 GPa for nominally pure TiB2 to 20.8 GPa for TiB2-15 vol.% SiC and a subsequent decrease with further increase in the SiC addition. Also the TiB2-30 vol.% SiC showed an improved indentation fracture toughness of 5.76 MPa m(1/2) which was mostly attributed to the addition of the SiC particles that could activate toughening mechanisms such as crack deflection, crack branching and grain breaking. The microstructure of the sintered samples was characterized by XRD and FESEM, which indicated that SiC reacted with and removed oxide impurities (TiO2 and B2O3) that were present on the particle surfaces. Elimination of such impurities promoted densification by minimizing grain coarsening and in composite contain 30 vol.% SiC the reaction led to in situ formation of TiC. Hence, the results indicated that sinterability of TiB2-based composites was remarkably improved by addition of SiC compared to the single phase TiB2 ceramic. (C) 2015 Elsevier Ltd. All rights reserved.
TiB2-SiC ceramic composites, with different contents of SiC whiskers (SiCw), as a ceramic sinter-additive, were prepared by the hot pressing process at 1850 degrees C for 2 h under a pressure of 20 MPa. For comparison, a monolithic TiB2 ceramic was also fabricated under the identical temperature, pressure, atmosphere, and holding time by the hot pressing process. The effects of fabrication process and SiC whiskers on microstructural features, phase evolution and mechanical properties were investigated. Hardness measurements revealed an initial increase in hardness for TiB2-SiC compared to TiB2. Also the improvement of the fracture toughness was attributed to the toughening and strengthening effects of SiC whiskers such as crack deflection. The results showed that promoted densification of TiB2-SiC ceramic composites is due to addition of SiC whiskers and reduction of oxide impurities by reacting with SiC whiskers and removing them from the surface layer of TiB2 particles. The reaction between TiB2 particles and SiC whiskers led to in-situ formation of TiC phase in the matrix as well. In general, it is concluded that the sinterability of TiB2-based composites was remarkably improved by introducing SiC whiskers compared to the single phase TiB2 ceramic. (C) 2016 Elsevier Ltd. All rights reserved.
Four nitride coatings, TiN, TiAlN, AlTiN and CrAlN were deposited on YG6 (WC+6wt.% Co) cemented carbide by cathodic arc-evaporation technique. The friction and wear properties were investigated and compared using ball-on-disc method at high speed with SiC ball as a counter material. The tests were evaluated by scanning electron microscopy, X-ray diffractometer, energy dispersive X-ray, micro hardness tester and an optical profilometer. The results showed that TiN and TiAlN coatings presented lower friction coefficient and lower wear rate, and that high Al content AlTiN and CrAlN coatings didn't present better anti-wear properties in this test. Oxidation and abrasive wear were the main wear mechanism of TiN coating. In spite of the observation of micro-grooves and partial fractures, TiAlN possessed perfect tribological properties compared with the other coatings. High Al content increased the chemical reactivity and aroused severe adhesive wear of AlTiN coating. CrAlN coating presented better properties of anti-spalling and anti-adhesion, but abundant accumulated debris accelerated wear of the coating under this enclosed wear environment.
Selective laser melting (SLM) is a powder bed fusion additive manufacturing (AM) technique that produces three-dimensional (3D) parts by fusing metallic powders with a high-energy laser. SLM involves numerous process parameters that may influence the properties of the final parts. Hence, establishing the effect of the SLM processing parameters is important for producing parts of high quality. In this study, titanium-tantalum alloy was fabricated by SLM using a customized powder blend to achieve in situ alloying. The influence of processing parameters on the microstructure and properties such as relative density, microhardness and surface roughness was investigated. The results show that fully dense titanium-tantalum parts can be obtained from SLM. With laser power of 360 W, scan speed of 400 mm/s, powder layer thickness of 0.05 mm and hatch spacing of 0.125 mm, the titanium-tantalum alloy produced by SLM has relative density of 99.85 +/- 0.18%. Despite the variation in process parameters, titanium-tantalum shows laminar beta grains in random directions in both xy and yz-plane from optical microscope (OM) analysis in all the parts produced. This observation is further confirmed using x-ray diffraction (XRD).