B. Lux

Hardness to toughness relationship of fine-grained WC-Co hardmetals

The hardness to toughness relationship of fine-grained WC-Co hardmetals was studied based on Palmqvist indentation toughness measurements. Sixty-five commercial and lab-sintered hardmetals of different composition, microstructure and manufacturing history were investigated to build up a representative hardness/toughness measurement band. This band is then used to discuss the influence of the various alloy- and process-related parameters on the hardness to toughness relationship of WC-Co composites. Beyond that, optimal hardness/toughness combinations can be assessed for the hardness range of 1400–2200 HV30. In general, the higher the hardness of the alloys, the longer were the indentation cracks, indicating a decrease in fracture toughness with increasing hardness. However, at a certain hardness, the toughness of individual alloys varied significantly. For example, at HV30:1670, the sum of crack lengths varied between 287 μm (high toughness) and 449 μm (low toughness), which corresponds to fracture toughness values of 11.5 and 9.2 MNm−32, respectively. Very fine-grained hardmetals (ultrafine grades) were shown to be not necessarily tougher than coarser grained alloys (submicron grades), in particular in the hardness range of 1450–2000 HV30, although they exhibit significantly more binder at a given hardness. Only in the high hardness range of > 2000 HV30 might they be of advantage. Samples, exclusively doped with Cr3C2 as growth inhibitor exhibit more favorable hardness/toughness combinations than comparable VC-doped alloys. However, other parameters, such as sintering temperature, sintering time, or the gross carbon content of the respective alloys must be taken into consideration for obtaining optimal hardness/toughness combinations.

Boron Nitrides — Properties, Synthesis and Applications

Boron nitride is a extraordinary topic in the area of materials science. Due to the special bonding behaviors of boron and nitrogen the BN exists in many different structures. The well-defined crystallographic structures are hexagonal BN (h-BN), rhombohedral BN (r-BN), wurtzitic BN (w-BN), and cubic BN (c-BN). Additionally, other crystalline and amorphous structures exist. Exceptional is that there are still discussions about the BN phase diagram. In the present stage c-BN is the stable phase at standard conditions but exact data about the phase transition line are not yet available. Synthesis of h-BN powders and coatings is described as well as applications of BN in ceramic materials and as lubricant. For c-BN the high-pressure high-temperature synthesis for powder production is discussed, and an overview about applications in wear resistant ceramics (polycrystalline c-BN) is given. The low-pressure methods for nano-cBN deposition (PVD and Plasma CVD) are described.

WC grain growth and grain growth inhibition in nickel and iron binder hardmetals

Abstract WC grain growth and growth inhibition of an 0.6 μm FSSS WC powder (average SEM size: 0.35 μm) were studied in WC–10 wt% Ni alloys by adding 0–2 wt% of inhibitor carbides (VC, Cr 3 C 2 , TaC, TiC and ZrC). Alloy gross carbon content turned out to be a crucial factor for WC growth in Ni alloys, even with high inhibitor additions. Coarsening was more pronounced in high carbon alloys, compared with low carbon grades, resulting in a significantly lower hardness. VC proved to be by far the most effective grain growth inhibitor in WC–Ni hardmetals, followed by TaC, Cr 3 C 2 , TiC and ZrC. Hardness increased with increasing amount of additive but reached a maximum above which it remained about the same. Experiments on WC–Fe–(VC) alloys revealed that WC grain growth is strongly restricted in Fe-binder alloys, even without additions of growth inhibitors. Binder chemistry thus strongly influences both continuous and discontinuous WC grain growth. This chemistry is determined by the nature of the binder matrix (Fe, Co, Ni), the alloy gross carbon content (which determines the composition of the binder matrix) as well as the inhibitor additive.

Characterization of ballas diamond depositions

Abstract Unfaceted, polycrystalline spherically grown diamond deposits having a radial structure have been observed since the early days of low pressure CVD diamond synthesis. Because the structure is quite similar to natural ballas stones, unfaceted CVD diamond is called ballas. So far, the general trend in diamond deposition has focused on well-faceted diamond layers, so CVD ballas deposits have not been systematically investigated. Low pressure growth of ballas always occur under conditions that are “non-optimal”, i.e. at least one parameter exceeds the range for a diamond growth leading to well-faceted diamond crystals. CVD ballas can consist of more than 99% of pure diamond; its microstructure reveals high amounts of micro-twins. Several morphological ballas structures have been observed by varying the deposition conditions, i.e. ballases having faceted areas, flat ballases, ballases with graphitic inclusions etc. Various deposits were characterized by Raman spectroscopy and impurities were measured by S IMS . Low pressure ballas diamond layers have a hardness quite similar to pure diamond. Of particular interest is the fact that cleavage and crack propagation along crystallographic planes can — due to the presence of micro-twins — be expected to be much lower in ballas than in single-crystalline diamonds. Thus, ballas structures are of particular interest for wear applications. Ballas type diamonds containing fine graphite particles could also be of interest for flat panel displays, as the graphite permits high electron emissions.

Internal stresses in CVD diamond layers

Abstract Chemically vapour deposited diamond layers always contain internal stresses. Two types of layer stresses can be distinguished: tensile intrinsic stresses and compressive thermal stresses. Intrinsic stresses were measured in situ during diamond deposition onto silicon substrates using a bending-plate method in a hot-filament reactor. The intrinsic tensile stresses increased with the total gas pressure and the methane content in the reaction gas. The measured stress curves became steeper with decreasing filament temperature. They also depended strongly on the layer morphology and grain size. The intrinsic stresses within a layer increased with decreasing grain size and the transition from well faceted crystallites to spherolitic ballas-type crystals having an extremely fine-grain radial polycrystalline structure. The thermal stresses, determined after cooling to room temperature, increase with higher deposition temperature and show acceptable agreement with calculated values using a simple model.

Deposition of ballas diamond and nano-crystalline diamond

Abstract Nano-crystalline materials are of high interest, because mechanical and physical properties of such materials are different from coarse grained types. In case of diamond the CVD ballas type is a nano-crystalline material. Other names for ballas are also common; e.g. ball-shaped diamond, cauliflower like, nano-crystalline diamond. Ballas is a non-faceted, polycrystalline spherically grown diamond having a radial structure. Several morphological ballas structures have been observed by varying the deposition conditions, i.e. ballas having faceted areas, flat ballas, ballas with graphitic inclusions, etc. The nano-crystalline diamond coatings were reported during the last years and grown by adding fullerens to the deposition gas or using Ar/CH 4 mixtures with less H 2 additions. From the present point of view the ballas morphologies can be described by various microstructures. The nano-crystalline diamond is one of these microstructures.

Investigation of the c-BN/h-BN phase transformation at normal pressure

Abstract The phase transition of c-BN into h-BN was studied up to 1540°C showing that c-BN is the stable modification at standard conditions. The transformation was examined by DTA (differential thermoanalysis) measurements. The morphology changes during conversion of c-BN into h-BN were investigated by SEM (scanning electron microscopy) indicating that the formation of h-BN preferably starts at the surface of the c-BN single crystals. The transformation step was furthermore characterized by IR and Raman spectroscopy as well as X-ray diffraction. The results have shown that a solid state and a gas phase mechanism have to be considered responsible for the phase conversion. The onset temperature of the phase transition from c-BN to h-BN depends significantly on grain size and impurities present in the samples. The lowest temperature for the beginning of the phase transformation was observed with a micron-size c-BN at 900°C.

Investigation of the boron incorporation in polycrystalline CVD diamond films by TEM, EELS and Raman spectroscopy

Abstract The incorporation of boron from B(C2H5)3 in the gas phase during the low pressure diamond deposition in a hot-filament reactor is investigated by various TEM techniques and Raman spectroscopy. Concentrations of B(C2H5)3:CH4 up to 3800 ppm were used in gas phase, which gives boron concentrations up to 3% in the layer according to SIMS measurements. The characteristic Raman peak of diamond at 1332 cm−1 remains nearly unchanged up to very high boron concentrations when single {100} facets are investigated. However, peak shift and drastic intensity decrease are connected with {111} facets. TEM shows diamond grains with defect densities comparable to undoped layers even for the highest boron concentration. Localized EELS spectra of this sample show the boron edge clearly for areas which can be attributed to growth on {111} facets by the defect configuration. This signal is absent for grain segments connected to growth on {100} facets, thus indicating inhomogeneous boron incorporation. Lattice dilatation of up to 0.3% is measured by CBED for crystal parts containing high boron concentrations. At the interface between these and boron free parts strain contrast can be observed because of the different lattice constants.

Influences of WC-Co hard metal substrate pre-treatments with boron and silicon on low pressure diamond deposition

Abstract Diamond coatings were produced on WC-Co hard metal substrates. To improve the adhesion between the diamond coating and the substrate a substrate surface pre-treatment with boron or with silicon vapor was applied. This surface pre-treatment resulted in an increase in both the diamond nucleation density and the diamond growth rate. Simple adhesion tests confirmed an improved adhesion of thin diamond layers as compared with those on untreated hard metal substrates. Secondary ion mass spectroscopy (SIMS) depth profiles revealed an enrichment of B or of Si at the substrate-diamond interface due to the pre-treatment procedure. The correlation of the Co and W depth profiles in samples coated for 12 and 24 h supports the theory of diamond dissolution into the substrate. Co was detected only in the interface regions and on the surface of the diamond layers but not in the bulk of the thick layers. The SIMS results confirm X-ray examinations of the hard metal Co binder phase.

Comparison of P, N and B additions during CVD diamond deposition

Abstract Using the hot-filament CVD method, the influences of phosphorus, nitrogen and boron additions on the growth mechanism of diamond films were investigated using phosphine, molecular nitrogen and triethylboron additions. The limits of diamond growth were investigated, revealing those domains in which the P, N, and B additions caused transitions from sp 3 to sp 2 or amorphous structures. Comparisons of the deposition results with thermodynamic equilibrium calculations suggest that activated species (i.e. HCP, HCN, resp. BH 2 ) induce a transition from faceted to unfaceted morphology. The observed changes of growth rate, morphology and crystallinity can be attributed either to surface or to gas phase reactions, and also to a reduction of the carbon supersaturation and defect-induced growth. It is shown that these transitions occur at concentrations that are similar for all three elements. The understanding of the mechanisms influencing the diamond growth and the incorporation of the elements P, N and B are important for doping experiments using these elements to produce semiconductive diamond.