Polycrystalline Diamond Grades Research Articles

Machining performance and wear behaviour of polycrystalline diamond and coated carbide tools during milling of titanium alloy Ti-54M

Polycrystalline diamond (PCD) is currently under development as a new generation of cutting tool material for titanium alloy machining applications. The unrivaled high temperature hardness possessed by PCD offers the potential for higher levels of productivity compared to tungsten carbide, the current industry standard tool material, through facilitating higher cutting speeds. This study investigates the performance of various PCD tool grades during square shoulder milling of Ti-54M. The influence of PCD grain size on dominant wear mechanism has been established, revealing that a smaller, sub 1 μm, grain size offers improvements in tool life due to superior fracture toughness compared to larger grained material. For fine grained PCD, loss of tool material through a cyclic process of workpiece adhesion followed by grain pull-out was identified to be the predominant wear mechanism, contrasting the mechanical fracture dominated wear observed for the larger grained PCD grades. The influence of insert microgeometry was also investigated through honing of the cutting edge radii. An increased tendency for edge fracture was demonstrated when machining with larger radii tooling which was attributed to increased cutting forces. Finally, the study has compared the surface integrity response of the workpiece following PCD and carbide machining, revealing considerably lower levels of microstructural damage and cutting forces when machining with PCD. This highlights the potential benefits of PCD in finishing applications, whereby high speed machining can be employed to reduce the impact on component surface integrity.

Small scale testing of PCD and WC-Co tooling in rock cutting using longitudinal turning

Rock cutting and crushing stands for a considerable part of the world use of Cemented Carbide (CC). CC is preferred due to its hardness while maintaining high fracture toughness. However, new cutting tool materials are developed allowing increased efficiency whilst minimizing the use of critical raw material such as tungsten and cobalt. One material showing promising capabilities is Polycrystalline Diamond (PCD).Field testing of new wear resistant tool materials are known to be notoriously difficult due to issues controlling the test environment. To further support the development of new and more wear resistant tool materials there is a need for an improved methodology of laboratory testing of rock cutting.In this work a test rig was developed and testing methodology was validated, when using cutting tool inserts with standard shapes, such as round double sided general purpose insert RNGN and dome shaped buttons while machining and crushing bars of red granite by longitudinal turning using a standard lathe. Optical microscopy, 3D data of Volume Difference Measurements and SEM were used for analyzing tool wear. Different grades of CC showed similar wear mechanisms irrespectively of tool geometry. The PCD grade did not show similar wear mechanisms, but did outperform CC grade in regard to wear resistance.

Effect of loading rate on the fracture toughness and failure mechanisms of polycrystalline diamond (PCD)

Quasi-static and dynamic fracture toughness tests are performed on four grades of PCD to evaluate the effect of rate and identify dominant failure mechanisms. The results indicate that the presence of the secondary phase cobalt has a significant negative influence on the fracture toughness of PCD grades at high rates of loading. An apparent rate insensitivity was exhibited for coarse grain specimen, where appreciable amounts of secondary phase cobalt were removed, or leached from the crack-tip region. The beneficial effect of cobalt removal on dynamic toughness was observed to be dependent on grain size, with fine grain microstructures exhibiting a similar negative rate sensitivity to corresponding non-leached grades. SEM analysis was used to evaluate the resulting fracture surfaces of both static and dynamic fractured specimens. The effect of rate on the prevailing fracture mode was analysed using a dynamic fracture mechanics approach.

Contact damage and residual strength in polycrystalline diamond (PCD)

The tolerance to contact damage of polycrystalline diamond (PCD) composites is investigated. Three different PCD grades are considered. Contact damage is extrinsically introduced by spherical indentation techniques, an approach well-established to evaluate damage tolerance issues in other hard materials. Such damage is induced on the surface of polished specimens, under both monotonic and cyclic loading conditions, and is characterized by optical and scanning electron microscopy. Residual strength is used as discriminative parameter for evaluation of damage tolerance. Hence, non-indented (reference) and indented specimens are fractured by means of biaxial ball-on-three-ball flexure testing. Results show that tolerance to contact damage is dependent on the microstructural characteristics of each PCD grade. Hence, although fine PCD grade exhibits the highest reference strength, corresponding indented specimens show pronounced fracture resistance decay for relatively high applied loads or number of cycles. On the other hand, the bimodal and coarse PCD variants are found to be more tolerant to contact damage, i.e. fracture strength shows only slight changes as cracks are introduced. The trends discerned are discussed on the basis of relative discontinuous character and sharpness of the induced damage, as well as effective depth of corresponding cracks, all of them directly dependent on the PCD microstructure assemblage.

Numerical analysis of the strength of polycrystalline diamond as a function of microstructure

Developing an understanding of the dominant failure mechanisms, and identifying the underlying structure–property relationship for polycrystalline diamond (PCD) are of fundamental importance for establishing a roadmap for the future design of materials with improved properties. In this paper, the Finite Volume (FV) method is used to generate representative PCD microstructures based on a diffuse-interface modelling approach. A cell-centred arbitrary crack propagation model is adopted to explore the strength and failure distribution of PCD as a function of the underlying microstructure. In particular, simulations are undertaken to identify the influence of phase morphology and individual constituent properties on the overall fracture strength of PCD. Results indicate that the morphology and distribution of the second phase cobalt relative to grain boundaries have a significant influence on the fracture strength of PCD. The stochastic nature of the strength of the individual grain boundaries was investigated. Larger variation in the distribution of interfacial bond strengths was found to result in a decrease in the overall strength for PCD grades. The effect of removing the cobalt from the microstructure was such that it reduced the overall strength but elevated the sensitivity to grain bond strength distribution. The presented work provides a framework for identifying the salient microstructural features, and offers a means for developing future grades of PCD through targeted virtual material design.

Tool wear behaviour and workpiece surface integrity when turning Ti–6Al–2Sn–4Zr–6Mo with polycrystalline diamond tooling

The paper details the performance of a range of polycrystalline diamond (PCD) tools (∼1.3–39μm average diamond grain size) when turning Ti–6Al–2Sn–4Zr–6Mo with 150bar cutting fluid. The alloy is used for aeroengine components such as turbine discs due to its superior mechanical and elevated temperature properties. Tool life improved from ∼30 to 80min with increasing grain size except with the ultra-coarse (∼39μm) PCD grade, which failed via chipping/edge fracture after ∼8min. In general, the principal wear modes were crater formation and workpiece adhesion. Workpiece integrity assessment showed no major subsurface damage with surface compressive residual stresses of ∼600MPa.

Characterization and Performance Evaluation of Mono-grain and Bi-modal PCD Grade in Machining of MMC

The aim of this investigation is to study the influence of different parameters such as grain size and cobalt content on the tool performance in the machining of metal matrix composite (MMC) using different polycrystalline diamond (PCD) tools. The experimental tests were conducted with four different PCD grades in dry the milling of Aluminium-Silicon carbide (Al-SiC) composite containing 20% wt of SiC as a workpiece material. The SEM study shows that PCD grades I and II have a bimodal grain size distribution. Rietveld refinement of the XRD spectra for these grades indicates that grade I contains larger amount of Co compared to grade II. The difference in performance between these grades can be explained by the Co content. PCD grade III is a coarse grain grade while grade IV is a fine grain grade. An XRD analysis shows that the main difference between the mono- grain and bimodal grain PCD is the presence of WC in the mono-grain grades which is believed to be added to the raw material of PCD as a grain growth inhibitor. In the XRD pattern for the bimodal grades the peaks in the WC phase are completely absent. The results indicate that the tool performance in terms of wear development is influenced by grain size distribution, cobalt content and the presence of WC; where the abrasive wear is the main wear mechanism which determines the tool life of the diamond grades in milling MMC. The results also indicates that the MMC can be machined with any diamond grade at the low range of cutting speed (≤ 400 m/min) while at a higher range of cutting speed (> 400 m/min) the selection of the diamond grade should be based on the chemical composition and grain size distribution. The presence of grain growth inhibitor also has a significant effect on the tool performance in machining MMC at cutting speeds above 400 m/min.

Machining MMC engineering components with polycrystalline diamond and diamond grinding

The high specific strength of metal matrix composite (MMC) materials is derived from the combined effects of light, ductile and hard, brittle materials being incorporated in a matrix composite. The hard, brittle phase in this composite can cause problems when machining such materials. The most commonly encountered problems are those involved in producing an acceptable surface finish, avoiding very rapid tool wear and achieving acceptable machining costs, through the use of higher machining speeds. However, in order for MMC materials to be widely accepted into the mainstream automotive, aerospace, and mechanical engineering industries, cost effective machining solutions will be required. Increasingly, machining with polycrystalline diamond (PCD) and grinding with diamond abrasives (two examples of ultra hard materials) are being utilised as the most effective machining methods in the manufacture of MMC components. The present paper explores the inherent problems involved in the machining of MMCs and the suitability of ultrahard tooling technology in overcoming many of these problems. The importance of PCD grade selection and optimised machining conditions are particularly important when machining MMCs, and these are reviewed in detail. The versatility of PCD for use in practically all metal cutting operations is also illustrated. The paper concludes with a number of case studies demonstrating how ultrahard tooling technology has been applied to produce economically a wide range of engineered MMC components in the automotive, aerospace, and mechanical engineering industries.

Diamond Grinding and PCD Machining of MMC Engineering Components

Many of the problems associated with the machining of metal matrix composites (caused by the presence of the hard, brittle phase) can be overcome by using ultrahard tooling. Tools made from polycrystalline diamond (PCD) are particularly suitable, however it is important that an appropriate PCD grade and optimised machining conditions are selected if rapid tool wear is to be avoided. This overview concludes with a number of case studies demonstrating how ultrahard tooling technology has been economically applied in a diverse range of engineering sectors.

Machining of Graphite/Epoxy Composite Materials With Polycrystalline Diamond (PCD) Tools

Machining of graphite/epoxy composite material is investigated by turning (cutoff) tests using different grades of polycrystalline diamond (PCD) inserts. The wear behavior of the PCD cutting edge is characterized by small cracks, rounded edges, and flank wear. The flank wear growth rate was found to depend on the microstructure of the tool and machining time. The coarser the PCD grade was, the better the wear resistance. The machined surface characteristics were evaluated by analyzing the surface roughness data and by scanning electron microscopy inspection of machined surface textures. Surface quality was found to get better with the cutting time.