Mechanical and Tribological Properties of Epitaxial Cubic Boron Nitride Thin Films Grown on Diamond

Abstract

Cubic boron nitride (c-BN) is a very promising material with respect to applications as a hard coating for cutting tools due to its many desirable properties including a hardness second only to diamond, oxidation resistance and chemical inertness against iron even at high temperatures with the latter property resulting in the ability to machine ferrous metals. However, these potential applications are hindered by the bad adhesion of c-BN films to most substrate materials. This tendency of c-BN films to delaminate also puts significant obstacles to accurate measurements of their mechanical properties. The poor adhesion of such samples is recognized as being due to the high compressive stress caused by energetic ion bombardment during growth and to a mechanically soft turbostratic boron nitride (t-BN) interlayer between c-BN and substrates. [1] Up to now, most of the c-BN films consist of nano-sized crystallites and have the above addressed layered structure. Thus, the elastic and mechanical properties, which have been studied with different methods, [2][3][4] are certainly not representative for c-BN alone. Recently, by introducing fluorine into the gas phase, thick polycrystalline c-BN films were synthesized by jet-plasma chemical vapor deposition (CVD) and microwave-plasma CVD [5,6] with a submicron lateral grain size and a nanoindentation study on these c-BN films was reported. [6][7][8] However, a 100 nm thick t-BN interlayer and the large number of grain boundaries still exist in these c-BN films. [7,8] Furthermore, in most cases sample polishing is required prior to hardness measurements since these c-BN films are considerably rough. [8] Very recently, we have demonstrated that thick heteroepitaxial c-BN films without any intermediate t-BN layer can be prepared on CVD diamond films and single crystal diamond substrates using ion beam assisted deposition (IBAD). [9,10] These thick, and single crystalline c-BN films possess very smooth surfaces that allowed to accurately determine their mechanical behavior as expressed by Young's modulus [11] and hardness.Scratch tests have been widely used as a convenient method for estimating adhesion of thin hard wear-resistant coatings such as titanium nitride (TiN), diamond-like carbon (DLC) and c-BN to substrates. [12][13][14][15][16][17][18] In the traditional scratch test procedure a diamond tip is drawn across the coating surface under a progressively increasing normal load until the coating becomes detached or fractured. The smallest load at which any recognisible failure occurs is called the "critical load" L c . However, the relationship between this critical load and the adhesive strength of the interface between the substrate and coating is yet unclear. In addition to the coatingsubstrate bond strength itself, a wide range of factors are known to affect the critical load value obtained from the scratch test. [19][20][21][22][23][24][25][26][27] The aim of this work was to show the relative improvement of the mechanical and tribological properties of the c-BN film...