Mechanical properties of multilayer pvd Ti/TiB₂ coatings

Abstract

To obtain from a tribological perspective acceptable PVD coatings of covalently bonded materials, such as borides and oxides, it is essential to control and often reduce the very high compressive stresses which develop during coating growth, since such stresses not only reduce coating adhesion but also limit film thickness. A possible way to obtain lower stress levels could be to utilise multilayer coatings, that is to combine the stressed material with a more ductile material in a layered structure. In the present study a highly stressed PVD TiB2 material was layered with ductile Ti. The multilayer period, i.e. the combined thickness of one Ti lamella and one TiB2 lamella, and the lamellar thickness ratio of the two phases were varied in order to evaluate their influences on some important mechanical properties and on the abrasive wear resistance of the Ti/TiB2 coatings. The coatings were deposited using magnetron sputtering of TiB2 and electron beam gun evaporation of Ti. To evaluate the coating phase composition X-ray diffraction was used. Mechanical and tribological properties were examined by nanoindentation (hardness and modulus), beam deflection (residual stress), scratch testing (cohesion and adhesion), and dimple grinding (abrasive wear resistance). Hardness, Young's modulus, compressive residual stress, and abrasive wear resistance were found to increase with the fraction of TiB2 in the coatings, and an inverse relation was found for the critical normal load obtained in scratch tests, i.e. cohesion/adhesion decreased with increasing TiB2 fraction. The investigation showed that a hybrid PVD technique combining electron beam evaporation of Ti and magnetron sputtering of TiB2 can be used to produce coatings with increased fracture toughness, decreased residual stress, and relatively high hardness as compared to homogeneous PVD TiB2. It was also found that the yield/fracture strength of Ti was insufficient, i.e. it could not accommodate the level of stress generated in the TiB2 phase. Therefore, by choosing another material with higher yield/fracture strength than Ti it is believed that the properties of the multilayer coating could be improved even further.

Microstructure Development in Multilayer TiBx/TiSiyCz Coatings during Post-Deposition Heat Treatment

“…• C of amorphous overstoichiometric TiB x coatings (of B/Ti ratio value in the range 2-2.45), formed by MS at room temperature condition, resulted in crystallization of equilibrium hexagonal TiB 2 phase as columnar grains of 5-10 nm in diameter, oriented along [001] direction [7,8], while excess boron formed thin layer (a tissue layer) separating TiB 2 grains. It was shown that this orientation was unchanged during annealing of the coating up to 800…”

“…Nanostructured coatings containing amorphous TiB 2 (a-TiB 2 ) were produced by plasmaenhanced CVD in Ti-B-N system [5]. However, coatings containing nanocrystalline TiB 2 phase were produced by PVD methods using bombardment of high-energy ions [7], a negative bias voltage applied to substrates [8] or by crystallization from amorphous phase. Depending on chemical composition of TiB x coating, deposition method, heat-treatment parameters, different temperatures of crystallization of TiB 2 phase are reported: in the * corresponding author; e-mail: atwardow@up.krakow.pl range 900-1100…”

Case Study: Glass‐Infiltrated Alumina

“…4 Recent research results on the possibility of improving of surface properties of wear -resistant materials suggest that deposition of coatings by several processes could be a potential solution. 10 However, large compressive stresses can cause failure of the coating by debonding, especially at coating edges. 9 An important factor is that high residual stresses are introduced during postdeposition cooling, which in turn affect the behavior of the coating.…”

Improvement in Adhesion of Sputtered TiB<SUB>2</SUB> Nano-Compostite Coatings onto High Speed Steel by a Chromium Interlayer

“…Similar techniques have been used to enhance the adhesion of TiB 2 coating onto steel substrates. 7,8) However, Berger and Larson 9) addressed that the yield strength and fracture strength of titanium interlayers were insufficient to accommodate the level of stresses generated in TiB 2 coatings during deposition and during application. In the present attempt to further improve the adhesion strength of sputtered TiB 2 coatings onto high speed steel (HSS) substrates, chromium was used as the interlayer material.…”