ICMCTF2010 Session AP: Symposium A Poster Session

Quaternary Si-B-C-N materials are becoming increasingly attractive due to their possible high-temperature and harsh-environment applications. In this work, hard (22 - 24 GPa) amorphous Si-B-C-N coatings were deposited on various substrates (Si, SiC, steel, WC and WC with 250 nm TiN or CrN interlayers) by pulsed dc magnetron sputtering using a single B₄C-Si target in a 50% Ar + 50% N₂ gas mixture. A fixed 75% Si fraction in the target erosion area, an rf induced negative substrate bias voltage of -100 V, a substrate temperature of 350 °C and a total pressure of 0.5 Pa were held constant during depositions. A planar rectangular (127 x 254 mm²) unbalanced magnetron was driven by a pulsed dc power supply operating at the repetition frequency of 10 kHz with a fixed 50% duty cycle and the average target power over a period of 470 W, being close to that used by us during a continuous dc magnetron sputtering of Si-B-C-N coatings [1-3]. Here, the aim was to avoid any discharge instabilities leading to possible defects in the coatings and thus, to improve quality of their surface. The high-temperature behavior of the coatings, including their oxidation resistance in air and thermal stability in inert gases up to 1700°C, was characterized by means of high-resolution thermogravimetry, differential scanning calorimetry, X-ray diffraction, Rutherford backscattering spectrometry and elastic recoil detection analysis. Pulsed magnetron sputtering of the B₄C-Si target in Ar atmosphere was performed prior to deposition at the duty cycle of 20%, the substrate temperature ranging from 350°C to 480°C and the substrate bias voltage of -1400 V for up to 30 min to enhance adhesion of the deposited Si-B-C-N coatings to various substrates.

[1] J. Vlcek, S. Hreben, J. Kalas, J. Capek, P. Zeman, R. Cerstvy, V. Perina, Y. Setsuhara, J. Vac. Sci. Technol. A 26, 1101 (2008).

[2] J. Capek, S. Hreben, P. Zeman, J. Vlcek, R. Cerstvy, J Houska, Surf. Coat. Technol. 203, 466 (2008).

[3] J. Kalas, R. Vernhes, S. Hreben, J. Vlcek, J.E. Klemberg-Sapieha, L. Martinu, Thin Solid Films 518, 174 (2009).

It is widely known, that many coating materials show very significant temperature dependence with respect to their mechanical properties, especially with respect to Young’s modulus and Yield strength or Hardness.

In the work is will be shown how dramatic the influence of this dependency on the mechanical performance of real coating-substrate systems could be. In order to avoid failure due to flawed stability and life time prediction by ignoring this material behavior it is very important to measure mechanical properties at high temperature.

By doing so, not only experimental difficulties must be overcome but also new concepts for the correct analysis and interpretation of the measured data are necessary. Meaning, the classical Oliver and Pharr method does not suffice. The authors will present the necessary extensions and how they have to be applied,

Experimental design is an effective method for conducting a reduced number of experiments in order to obtain the optimum spraying conditions and enhance the thermally sprayed coatings properties. In the present study, a 2³ factorial design experiment was used to establish the effects of the variables on the coatings quality in relation to the coating microstructure (porosity and hardness). Response surface methodology (RSM) was employed to describe the empirical relationships among variables such as the arc current, the arc voltage and the powder feed rate. The maps obtained allowed the selection of the optimum operating conditions able to achieve the desired microstructural characteristics of these coatings deposited by APS. The analysis of the results indicates that the powder feed rate have the higher significant effect on both porosity and microhardness of these coatings.

In recent years under certain operating conditions calcium, magnesium, alumino silicate (CMAS) attack of thermal barrier coatings (TBCs) has become a concern. CMAS attack is essentially a high temperature corrosion process which initiates at temperatures above 1240°C, the temperature at which CMAS, deposited on turbine blades during operation, melts and infiltrates the open columnar structure of the thermal barrier coating. The mechanism of attack, which involves the dissolution of the TBC and the re-precipitation on cooling, is fairly well understood.

This paper looks at the effects of CMAS attack on erbia doped electron beam (EB) physical vapour deposited (PVD) TBCs and correlates it to previous work on standard 8wt% yttria partially stabilised zirconia. The effects of time and temperature on the degradation of the coatings have been examined using a combination of scanning electron microscopy, Raman spectroscopy and XRD analysis. Temperatures of 1250°C to 1400°C for 1-12hrs have been studied for different initial CMAS deposit concentrations and the depth of penetration and the degree of phase transformation, as a result of both leaching and re-precipitation, have been studied. Finally the concept of a “safe” limit of CMAS deposition as a function of time and temperature is also discussed.

The high temperature oxidation performance of nanocomposite coatings becomes an important issue recently. In this study, the ZrN and Zr-Si-N nanocomposite thin films have been prepared by a bipolar asymmetric pulsed DC magnetron sputtering system. The thin films with silicon content ranging from 2.2 to 10.4 at.% were prepared by adjusting the Si target power. An amorphous and featureless microstructure was observed on the coating with high silicon contents. A thermo gravity analyzer was adopted to study the oxidation resistance of thin films at 900^oC in static air. Surface and cross-sectional morphologies of coatings before and after oxidation tests were examined with a field emission scanning electron microscope, respectively. The crystalline phases of thin films were also analyzed with an X-ray diffractometer. It is observed that the high temperature oxidation resistance of Zr-Si-N thin films is better than that of pure ZrN coating. In general, the high temperature oxidation resistance of Cr-Si-N nanocomposite thin films increases with increasing Si content. The microstructure and phase evolutions were discussed in this work.

First stage of the research consists in optimizing the processing route to generate homogeneous microstructure and controlled surface roughness. The objective is to reduce as much as possible the size and depth of the surface cracks network inherent to the process. Indeed, the durability of the TBC when cyclically oxidized strongly depends on the sharpness of those cracks that concentrate thermomechanical stresses and generate detrimental propagation resulting in spallation.

Cyclic oxidation tests are performed using a newly developed equipment able to establish controlled thermal gradient through the thickness of the TBC and instrumented with CCD cameras to monitor in a real time basis the mechanism of crack propagation and spallation. The impact of various parameters, either directly related to the processing route, e.g. the intimate microstructure of the TBC and the TBC thickness, or to the thermal loading, e.g. the magnitude of the through thickness thermal gradient, the oxidation temperature and the cumulated hot time, on the durability of the TBC is investigated.