ICMCTF2005 Session B7-1: Properties And Characterization of Hard Coatings And Surfaces

Monday, May 2, 2005 10:30 AM in Room Golden West

Monday Morning

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10:30 AM B7-1-1 Design Methodology for Optimized Die Coatings: The Case for Aluminum Pressure Die-Casting
J. Lin, A.O. Kunrath, D. Zhong, S. Myers, B. Mishra (Colorado School of Mines); P. Ried (Ried and Associates); J.J. Moore (Colorado School of mines)

The paper will present the methodology used in the Advanced Coatings and Surface Engineering Laboratory (ACSEL) of the Colorado School of Mines to design optimized die coatings employed in material forming processes in an effort to extend the life and effect efficient operation of the dies, with dies used in aluminum pressure die casting being used as the specific example.

An optimized die coating 'architecture' requires that the coating system be essentially non-wetting with the material (metal, glass, polymer) being formed in the die, coupled with good wear and oxidation resistance Other factors, such as delaying the onset of thermal fatigue cracking (heat checking), and an acceptably low coefficient of friction. (including self lubrication) also need to be considered based on the processing and forming conditions that include both liquid and solid materials. Many different die coatings have and are being used with different levels of success. This paper presents the current understanding that has been gained in laboratory testing, in-plant trials, and modeling in an effort to generate a fundamental understanding of how such optimized die coating systems may be designed for the aluminum pressure die-casting industry.

11:10 AM B7-1-3 Structure and Mechanical Properties of TiC/Cu Films Prepared by DC Magnetron Sputtering
J. Soldan, J. Musil (University of West Bohemia, Czech Republic)
TiC/Cu nanocomposite films with various copper contents were deposited by dc unbalanced magnetron sputtering from an alloyed TiC target fixed by Cu rings of different inner diameters in pure argon. This makes it possible to prepare TiC/Cu films with Cu content ranging from approximately 4 to 80 at.% Cu. The structure and mechanical properties were examined as a function of deposition parameters and Cu content in the film. In deposition of TiC/Cu films the following deposition conditions were used: magnetron discharge current Id=1A, substrate bias Us=-50 and -100 V, substrate ion current density is ranging from 0.3 to 0.8 mA/cm2, substrate temperature Ts=500°C, substrate-to-target distance ds-t=50 mm and argon pressure p=1 Pa. It was found that a grain size of the nanocomposite decreases with increasing Cu content from 4 to 25 at. %. The further increase of Cu content in TiC/Cu films above 30 at. % Cu, however, results in increase of the grain size and formation of grains with preferred Cu(111) crystallographic orientation. The hardness of TiC/Cu nanocomposite decreases with increasing Cu content in the film from ~36 GPa for a pure TiC films to ~2 GPa for TiC/Cu film with 80 at.% Cu. The TiC/Cu nanocomposite film with the lowest (4 at. %) content of Cu exhibits a maximum hardness of approximately 26 GPa. The addition of Cu into TiC film results in dramatic decrease of the compressive macrostress σ generated in the film during its growth. While σ in the TiC film slightly exceeds -3GPa, that in the TiC/Cu composite film with 4 at. % of Cu is considerably lower, approximately -0.8 GPa. Next increase of Cu content in the film results in further reduction of σ and TiC/Cu films with highest (~80 at. %) Cu content exhibit even a tensile macrostress σ of approximately +0.1 GPa.
11:30 AM B7-1-4 Sputter-Deposited Boron Carbide Films: Structural and Mechanical Characterization
L.G. Jacobsohn, M. Nastasi (Los Alamos National Laboratory)
Amorphous boron carbide (B4C) films were deposited on (100) Si by dc-magnetron sputtering with an Ar working pressure of 5 mTorr at room temperature. The substrate bias (Vb) was varied between 0 and -200 V. Chemical analysis of the films was carried out by Rutherford backscattering spectrometry (RBS), x-ray photoelectron spectroscopy (XPS) and nuclear reaction analysis (NRA) and revealed the samples to be stoichiometric with negligible oxygen contamination. Higher Vb values lead to higher defect concentrations, as shown by positron annihilation spectroscopy (PAS) measurements, and higher amounts of trapped Ar atoms. Monte Carlo simulations of Ar bombardment of B4C suggest these defects to be ascribed as vacancies. While there is no correlation between the defect concentration and mechanical properties, a clear indication that trapped Ar deteriorates the hardness of the films was found. Moreover, hardness increase was obtained by the creation of new intericosahedral chains induced by post-deposition annealing as revealed by infrared and Raman measurements.
11:50 AM B7-1-5 Nanostructure and Performance Evaluation of Sputtered TiAlB Thin Films
C.G. Rebholz (University of Cyprus); M.A. Baker, M. Monclus (University of Surrey, United Kingdom); A. Leyland, A. Matthews (The University of Sheffield, United Kingdom); J.M. Schneider (RWTH Aachen University, Germany)
TiAlB films were deposited by co-sputtering from TiAl and TiB2 targets onto Si (110) and AISI 316 substrates. The influence of chemical composition and microstructure on mechanical and tribological properties was investigated. For various TiAlB compositions the stoichiometry and relative phase composition were determined using X-ray photoelectron spectroscopy and the microstructure and grain size were examined by transmission electron microscopy. Evidence of Al, Al3Ti, Ti and TiB2 phase formation was observed. Correlation of compositional and mechanical data showed that the elastic modulus and film hardness increased approximately by a factor of two and three respectively with increasing B/Al ratio over the selected range, due to a change in microstructure and grain size. Predominantly metallic coatings with moderately high hardness (ie. 15-25GPa), and an elastic modulus closer to that of the substrate material (ie. ≤ 250GPa), appear to perform well in terms of resistance to a variety of wear processes (sliding, impact, abrasion, etc.) as a result of being highly elastic and resilient (ie. possessing high H/E ratios).
12:10 PM B7-1-6 The Reduction of Surface Discontinuities for Cr-DLC Coatings Deposited using a Reactive Sputtering Process
B. Feng, M.A. Taher, H.K. Yoon (Caterpillar Inc.)
Uncontrolled surface arcing in reactive sputtering processes is believed to be a potential root cause for surface discontinuity generation in Me-DLC coatings, which could impact the failure mode of these coatings in tribological applications. In this study, the evolution of surface discontinuities, hardness, modulus and growth rate of a chromium-containing diamond-like carbon (Cr-DLC) coating was investigated using a special substrate shutter. A closed field unbalanced magnetron sputtering apparatus with two Cr targets was used. Three arc-handling modes of the substrate bias DC power supply were tested in separate depositions with the same Cr-DLC coating recipe. DC cathode power supplies were operated at regular mode. Using a consistent graphical analysis routine to characterize the optical image of coating surface discontinuities, the results show that the level of surface discontinuities depends on the arc-handling mode selected. A significant reduction in the level of these discontinuities can be achieved by using a pulsed substrate bias. The tests also show that the pulsed bias has no impact on the hardness, modulus or growth rate of Cr-DLC coatings. Additional tribological testing indicated that wear failure by micro-scale fracture and delamination might be correlated to the amount of surface discontinuities.
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