David J. Rowcliffe

Compressive deformation and fracture in WC materials

Plastic flow and fracture have been studied in WC single crystals and in WC-Co materials. Deformation was introduced by several techniques which resulted in the development of high compressive stresses which encourage slip in WC. Optical and transmission electron microscopy studies show that plastic flow in the carbide phase always precedes fracture. A simple analysis shows that there are only four independent slip systems in WC. Consideration of the limitations for slip in WC lead to a mechanistic model for fracture initiation in WC-Co.

Defect interactions in deformed WC

Abstract Reactions between the leading partial dislocations of extended defects lying on intersecting slip planes are observed by transmission electron microscopy (TEM) in samples indented at room temperature and 1000°C. The resultant dislocation is shared by the original defects. Annealing experiments performed at 1000°C for 5 hours indicate that the new configurations are stable and sessile and therefore can limit plastic flow. These reactions may also lead to crack nucleation via a mechanism similar to that proposed by Cottrell for b.c.c. metals. This mechanism may explain the observation of cracks on {11 2 0} and {1 1 00} planes generated at slip band intersections.

Intergranular cracking in WC-6% Co: An application of the von mises criterion

Abstract Intergranular cracks between WC grains in the plastic zones of indentations in WC-6% Co have been analyzed using transmission electron microscopy. The primary slip systems in the carbide are of the type {1100} 〈1123〉. Such a combination can generate only four independent strains, one less than the five required for compatible plastic flow in a polycrystalline aggregate. In particular, WC is incompressible along [0001]. A model is proposed to predict the likelihood of the failure of a grain boundary, given both its crystallographic orientation and those of the two adjoining grains. The experimental evidence on both cracked and uncracked grain boundaries bears out the theoretical predictions, i.e. that cracking is a consequence of large normal stresses acting on a boundary and arising from the incompatible deformation of neighboring grains.

Plastic Deformation of Transition Metal Carbides

The transition metal carbides possess a combination of properties1 that places them in a unique position in any classification of compounds. Both the elastic moduli and the melting points are extremely high which are primary characteristics of covalent solids. On the other hand, some of the electrical properties of the carbides are similar to those of metals although some important differences have been described that reflect other contributions to the bonding.2 Close to roam temperature slip and cleavage occur on the same planes as in ionic solids with the same crystal structure, but at high temperatures the details of slip show great similarity to those of metal crystals. This multifaceted character makes it particularly difficult to predict and understand plastic deformation processes in the transition metal carbides. Individual observations can be explained but the details of deformation are not as well understood, at the fundamental level, as they are in a material such as silicon.

A New Catalytic Method for Producing Preceramic Polysilazanes

A transition metal (e.g., Ru 3 (CO) 12 , Pt/C) catalyzed process for Si-N bond formation is discussed that provides a new route to mono-, oligo-, and polysilazanes. The catalysts function by activating Si-H bonds in the pres-ence of ammonia. Polymeric silazanes can also be produced from oligomers in the presence of ammonia at low temperatures. This method allows us to control or modify the composition of the polysilazane during or after the polymeriza-tion. A variety of polysilazanes were prepared and converted to Si 3 N 4 with ceramic yields ranging from 55%-85%. By varying the monomers and reaction conditions, we can control the nitrogen and carbon content in the preceramic polymers, which enables us to obtain ceramic products that are primarily Si 3 N 4 and simultaneously minimizes the coproduction of SiC and C.

Deformation enhanced decarburization of WC-Co

The paper describes an unusual transformation that takes place only within the deformed region of identations. Under a suitable ambient temperature decarburization of cemented WC-Co on annealing is enhanced by prior localized deformation. Within individual WC grains, plate-like growth of a mixed carbide (either Co/sub 3/W/sub 3/C or Co/sub 6/W/sub 6/C) occurs preferentially. On some occasions intrusion of the substoichiometric carbide is preceded by a structural transformation in the WC slip band. This transformation is consistent with the formation of an orthorhombic (pseudo-hexagonal) WC phase that may be derived from the original structure by the passage of one partial dislocation on every sucessive slip plane.

Defect Structure of WC Deformed at Room and High Temperatures

Single crystals of WC, deformed by micro-indentation at room temperature and at 1000°C, are examined by transmission electron microscopy in order to determine the mechanism of slip. The plastic deformation induced by indentation occurs by the motion of partial dislocations with Burgers vectors 1/6 \( \left\langle {11\bar 23} \right\rangle \). These partial dislocations combine in pairs to form extended dislocations with Burgers vectors 1/3 \( \left\langle {11\bar 23} \right\rangle \). Deformation in samples indented at 1000°C takes place by the same mechanism.

Dip process thermal barrier coatings for gas turbines

A new concept to apply zirconia‐based thermal barrier coatings on cobalt base alloys has been developed. Contrary to plasma spraying or electron beam vaporization, the new process produces a dense and highly adherent zirconia coating that resists thermal cycling and penetration by corrosive molten salts. The new method is based on thermally growing a ZrO2‐based layer from a Zr‐rich alloy, predeposited on a Y‐rich substrate by hot dipping. The coating consists of an outer ZrO2/Y2O3 layer and an inner oxide–metal composite layer next to the substrate surface. The outer oxide layer acts as a thermal barrier, while the inner layer acts as a graded seal that improves the adhesion of the coating to the substrate. Thermal cycling experiments showed that the coating has a good resistance to spallation between room temperature and 1100 °C.

Indentation Damage in Tungsten Carbide and Tungsten-Titanium Carbide

Tungsten carbide (WC) is the main constituent of cemented carbides used for metal machining and rock drilling. Titanium carbide (TiC) is added to some grades of WC-Co to improve wear resistance in cutting metals. These grades contain both hexagonal WC and cubic (W, Ti)C solid solutions as the principal hard phases. In both applications high stresses and high temperatures are generated locally at the tool surface by contact with asperities or with rock fragments. Under normal rock-drilling conditions the cutting surfaces can reach temperatures up to 400°C1 and temperatures as high as 800°C might be reached with dull bits or under unusually high loads.2 Tool-tip temperatures up to 1000°C3 have been recorded in metal cutting operations.