Krishna Valleti

Processing-structure-property relationships in electron beam physical vapor deposited yttria stabilized zirconia coatings

The physical and mechanical properties of yttria stabilized zirconia (YSZ) coatings deposited by the electron beam physical vapor deposition technique have been investigated by varying the key proc ...

Influence of substrate temperature and bias voltage on properties of chromium nitride thin films deposited by cylindrical cathodic arc deposition

Chromium nitride (CrN) thin films have been deposited on high speed steel (HSS) substrates using rotating cylindrical cathodic arc deposition technique and the influence of substrate temperature (TSub) and bias voltage (VSub) on the physical/mechanical and corrosion resistance properties of the films comprehensively investigated. An increase in TSub was found to significantly influence the phase composition of films, which changed from a mixture of Cr + CrxN + CrN to predominantly CrN. This was also accompanied by an increase in droplet formation and columnar grain size. With increase in TSub over the range investigated (230 °C to 500 °C), an increase in adhesion strength by nearly 30% was observed. In contrast, change in VSub from −50 to −150 V resulted in the growth of highly dense (111) oriented CrN thin films but with relatively little change in phase constitution, adhesion strength or microdroplet formation. A change in TSub from 230 °C to 500 °C was accompanied by a nearly 50% fall in corrosion resi...

Effect of arc suppression on the physical properties of low temperature dc magnetron sputtered tantalum thin films

Arcing is a common phenomenon in the sputtering process. Arcs and glow discharges emit electrons which may influence the physical properties of films. This article reports the properties of tantalum (Ta) thin films prepared by continuous dc magnetron sputtering in normal and arc-suppression modes. The substrate temperature was varied in the range of 300–673K. The tantalum films were ∼1.8μm thick and have good adherence to 316 stainless steel and single-crystal silicon substrates. The phase of the Ta thin film determines the electrical and tribological properties. The films deposited at 300K using both methods were crystallized in a tetragonal structure (β phase) with a smooth surface (grain size of ∼10nm) and exhibited an electrical resistivity of ∼194μΩcm and a hardness of ∼20GPa. When the substrate temperature was 473K and higher, the arc-suppression mode appears to influence the films to crystallize in the α phase with a grain size of ∼40nm, whereas the normal power mode gave mixed phases β and α beyon...

Cralsin Nanocomposite Thin Films for High Speed Machining Applications

ABSTRACT CrAlSiN nanocomposite thin films with varying film chemistry were developed on tungsten carbide (WC) specimens using cylindrical cathodic arc physical vapor deposition (c-CAPVD) technique. The physical, mechanical, and tribological properties of all the films were comprehensively investigated for arriving at the film chemistry leading to the best properties with respect to mechanical applications. The best tribo-mechanical properties were obtained in films with Cr/(Al+Si) ratio of 1.2. This coating with best properties was translated on to WC drill bits for machining tests. The Al and Si content has shown major influence on the adhesion strength and phase constitution of the films, with a considerable change in residual stress too. The superior properties achieved could be attributed to the formation of a near-perfect nanocomposite structure, with the crystalline CrAlN phase surrounded by an amorphous Si3N4 phase. The tool life of the coated CrAlSiN tools was investigated during dry machining of EN 24 material. In comparison to the tool life of an uncoated tool and a TiAlSiN-coated tool, the best CrAlSiN coatings synthesized in this study performed exceedingly well. The present study clearly demonstrates the advantages of CrAlSiN over other existing similar coatings for high-speed machining.

Studies on hard TaN thin film deposition by R C-Mag technique

The physical and mechanical properties of pulsed rotating cylindrical magnetron sputter-grown tantalum nitride (TaN) thin films were studied. Initially, films were grown at ambient substrate temperature by varying the reactive (N2) to sputter (Ar) gas ratio (R) at a constant pulsing frequency of the target power (100kHz). The results were compared with planar magnetron-grown TaN samples. The R C-Mag. grown thin films have properties nearly similar to the high temperature (300°C) dc planar magnetron sputter deposited samples. In comparison to the planar magnetron deposition, the progression of the phase composition occurs over a wider range of R in the pulsed R C-Mag. deposition. These observed differences for R C-Mag. deposition are attributed to the increased glancing angle deposition of adatoms and pulsing of the target power. To study the effect of pulsing frequency of the target power in R C-Mag., the films were also grown at different frequencies at a fixed R (0.1). With the increase in frequency, th...

Studies on cathodic arc PVD grown TiCrN based erosion resistant thin films

Titanium chromium nitride (TiCrN) coatings with varying Cr content in two configurations, mono- and multilayer, were deposited on high speed steel substrates using a cylindrical cathodic arc physical vapor deposition technique. The physical, mechanical, and erosion behavior of the coatings were investigated. Among the monolayer coatings, the thicker Ti0.48Cr0.52N coatings yielded the best erosion resistance property. But with the increase in thickness, a considerable increase in residual stress is observed. Toward minimizing the stress accumulation, the effect of multilayering with periodic in situ heat treatment (after each 1 μm film growth) was studied by growing films in Ti0.52Cr0.48N/Ti0.40Cr0.60N bilayer configuration. A new approach based on % area of erosion damage for measuring relative wear rate of thin films has been proposed and implemented. The multilayer coatings exhibited superior erosion performance compared to the well-known erosion resistant TiN coatings that are in use for compressor bla...

Structure-Property Correlations in Cathodic Arc Deposited TiAlN Coatings

PVD hard coatings, notably transition metal nitrides and carbides, are being increasingly used by industry for improving the life and machining speeds of cutting and forming tools. There has been an increasing trend towards use of complex coatings, based on ternary and even more complex multi-component systems, as well as in novel configurations such as multilayers, superlattices, nanolayers and graded coatings, to achieve superior properties in the tool as well as the finished product. The service properties of the coatings are known to be influenced by their microstructure, phase assembly and composition, apart from the orientation and stress states which can be suitably tailored for diverse applications. In the present study, a ternary coating based on Titanium Aluminum Nitride was deposited on high speed steel substrates by cathodic arc evaporation under varied bias voltage conditions. As-deposited coatings were characterized by X-ray diffraction, Residual Stress Analysis, Scanning Electron Microscopy (SEM), EBSD and FIB. Mechanical and tribological characteristics of the coatings were evaluated by nanoindentation and nanoscratch testing, respectively. The variations in coating hardness and adhesion with the bias voltage were studied. The changes in coating microstructure as a consequence of variation in bias voltage were also examined. Results from the above investigations are presented to illustrate how a combination of electron microscopy with nanoindentation and adhesion testing can be utilized to ascertain structure-property correlations in coatings.