Coarse-grained alumina is among those monolithic ceramics which can exhibit an increase in crack resistance with crack extension. This R-curve behavior is most pronounced for intergranular fracture and does not depend exclusively on material properties. Crack and specimen geometries also influence the shape of the R-curves. The magnitude of the effect increases with increasing crack surface roughness, which is microstructure-dependent, and with crack-opening displacement, which is geometry-dependent. Based on experimental observations, a “dynamic”R-curve model is presented which relates the increasing resistance to an increasing crack tip shielding caused by crack surface bridging. Applying a J-integral approach, R-curves are calculated for two specimen geometries (short double cantilever beam and single-edged notched beam) and different grain sizes. The good agreement between calculation and experiment indicates that the R-curve behavior of long cracks in alumina can be predicted by a simple wake model.

R‐Curve Behavior of Long Cracks in Alumina

A review of diamond synthesis by CVD processes

Tool wear and its mechanism for cutting SiC particle-reinforced aluminium matrix composites

Chemically vapour deposited diamond coatings on cemented tungsten carbides : Substrate pretreatments, adhesion and cutting performance

In the last 30 years, crack growth resistance curves (R curves) have been measured and analyzed for many ceramic materials. This body of literature is reviewed critically to address the utility of these measurements. Three effects have been held responsible for the existence of a rising R curve: crack face interaction in the wake of the crack tip, microcracking and crack branching ahead of the tip, and phase transformation ahead of the crack tip. For the crack face interaction, the relation between bridging traction and crack opening displacement characterizes the material behavior. Most experimental investigations have concentrated on specimens with macrocracks and on indentation-induced cracks. Very few results for R curves starting from natural flaws have been published. From these results, it is concluded that the behavior of specimens with natural flaws cannot be predicted directly from the results of tests with macrocracks and no established framework exists to correlate the two. The analysis of indentation cracks contains many uncertainties and cannot close the gap between macrocracks and natural flaws. The effect of a rising R curve on the strength and on the scatter of the strength is also discussed in the paper.

What Can We Learn from R‐Curve Measurements?

Pretreatment of substrate surface for improved adhesion of diamond films on hard metal cutting tools

The fracture properties of a series of three polycrystailine aluminas are examined using indentationstrength and double cantilever beam techniques The indentation-strength response is shown to be sigmoidal with concave-down behavior at small indentation loads and concave-up behavior at large indentation loads A model is developed for the general response, combining increasing toughening by ligamentary bridging at small crack lengths and increasing residual-stress relief by lateral cracking at large indentation loads The model is fit to the strength data and used to deconvolute the underlying toughness variation and predict the intrinsic strength of the materials Direct measurements of toughness using the “long-crack” double cantilever beam geometry are shown to overestimate the toughness variations effective during “short-crack” strength tests

Sigmoidal Indentation–Strength Characteristics of Polycrystalline Alumina

In order to improve the wear protection of hardmetal cutting tools, polycrystalline diamond coatings with high quality have been produced by the use™-process The strong adhesion of the diamond coating by mechanical clamping is generated by the pre-treatment of the WC-6 wt% Co substrate surface The wear behaviour of micro- and nanostructured crystalline diamond films was examined with cavitation test The influence of diamond film structure (morphology, crystal size and orientation, roughness) as well as coating thickness and adhesion at the interface on the wear mechanisms was investigated Regarding lifetime and performance of the diamond-coated tools, the nanocrystalline diamond structure shows a very high potential in industrial machining applications The nanostructured diamond with its smooth surface surpasses the microstructured diamond as coating material for suitable substrates with even complex pre-shaped geometries The extraordinary wear resistance for thin (3 μm) smooth films with optimised layer strength and fracture toughness could be exceeded by further increasing the coating thickness

Diamond films for wear protection of hardmetal tools

Enhanced nucleation of diamond films on pretreated substrates

Polycrystalline diamond films were synthesised on WC-Co hard metal tools by DC plasma-jet CVD. For machining abrasive materials the hard, wear resistant diamond films must be of good quality (roughness, homogeneity, crystallinity, structure) and show strong adhesion to the substrate. The wear behaviour of thin diamond films has been examined with abrasive particles in blast wear test and under ultrasonically-induced high frequency vibrations of a sonotrode in cavitation tests. The conditions of CVD diamond synthesis were varied with regards coating duration and process engineering. The influence of pre-treating the substrate surface for enhanced adhesion of the diamond film was also analysed. The microjets of the cavitation test react sensitively to coating defects considering the microstructure of the material. The failure mechanism (adhesive, cohesive) could be deduced from the worn surface. The blast wear test is less sensitive to coating microstructure, but the time of first coating failure as a quantitative wear criterion is helpful for improving the quality and adhesion of the diamond films. Thereby, the adaptation of synthesis conditions of diamond films is supported.

F. Deuerler

Papers

Pretreatment of substrate surface for improved adhesion of diamond films on hard metal cutting tools

Sigmoidal Indentation–Strength Characteristics of Polycrystalline Alumina

Diamond films for wear protection of hardmetal tools

Enhanced nucleation of diamond films on pretreated substrates

Wear mechanisms of diamond-coated tools