When loss of material due to sliding of two solids is promoted/prevented, in the presence of chemically reacting liquid or gas, tribochemical wear is said to occur. Tribochemical wear, in which corrosive media promotes material loss, is a serious concern in a variety of applications like machining, bio-implants, gas turbine engines etc. The most pervasive corrosive media encountered in applications are water and air. Air also contains water vapour along with oxygen, both of which adsorb and react with most materials, thus influencing their wear behaviour. The need for higher operating temperatures and compression ratios in gas turbine engines require development of high temperature wear resistant coatings to protect their soft metallic components. Ti based nitride coatings with Ti, Al, Si, Cr, Ta, Nb, V are known for wear resistance because of their high hardness which is second only to diamond and c-BN. High O affinity of these elements, induce the coatings to form passive oxide scale up to reasonably high temperatures and offer superior corrosion and oxidation resistance. However, sliding can remove the passivating layer, exposing the native surface to the environment which can lead to enhanced tribochemical wear. Oxidation resistance under static conditions does not guarantee low tribochemical wear; however, the tribochemical reactions causing the corrosion are of interest. Another concern is that sliding in unison with high temperatures can activate processes like enhanced diffusion, phase transformations in nitride coatings as well as in the substrate. Hence one of our objectives is to perform wear tests at high temperatures to understand the dominant mechanisms that affect wear in nitride coatings. Wear tests in the range of room temperature up to the oxidation limit of these coatings are designed.In this study TiN and high aluminium containing TiAlN coatings are chosen to study understand the wear behaviour as function of temperature up to 800°C [1]–[3].
In order to study wear of coatings, it is necessary to identify the best possible materials and methods. Though under the targeted application the coatings have to perform under fretting tests, pin on disk configuration is used which simplifies the analysis and gives deeper insight into the wear mechanism. Coated ball is used as the pin which is stationary unlike many earlier studies where the coating is applied on the rotating disk. The purpose of keeping the pin stationary is to minimize the counter-face wear and, instead, accelerate wear in these hard coatings. This method also enables easy and accurate measurement of wear depth and volume by using an optical microscope, while the conventional coated disk method requires profilometry and statistically sound measurements. To enable coating performance, substrate should not undergo much loss of strength before 800°C and hence aerospace grade IN718 alloy is chosen as the substrate which softens slowly beyond 650°C. Alumina is used as counter-face, since it has high hardness, excellent mechanical, chemical and thermal stability.
In the current study, TiAlN coating is tested for wear in the range of room temperature to 800°C. Figure 1 represents the data obtained from the wear experiments. It is found that the wear is higher with large scatter at room temperature while it remains constant from 200- 750°C. Two important observations are made, firstly that the TiAlN is susceptible to some kind of a corrosive wear at room temperature which depended on the timing of the tests and secondly that the coating shows a surprisingly constant wear behaviour over the temperature range of 200-750°C.
The scatter at room temperature is found to be linked with seasonal fluctuation of humidity which is verified by performing tests under controlled humidity conditions. Water vapour and oxygen are potential reacting gases present in air. Oxidation and oxidative wear is known to occur in many materials as temperatures increase which seem to be linked to thermal activation of oxidation. However lower wear at 200°C and above compared to room temperatures suggests something else to be happening .It is evident then that between room temperature and 200°C lies a transition of some kind in the tribochemical reaction which is responsible for the observed wear behaviour of TiAlN. A detailed study to understand this transition is then undertaken for the composition of TiN coatings so that benchmarking and comparison with TiAlN is possible. Also if the wear behaviour of TiN is found similar to TiAlN then it would indicate a general phenomenon which can be extended to Ti based nitrides.
Figure 1 Wear rate as a variation of temperature for TiAlN coatings
In contrast to low temperature wear behaviour of TiAlN, a constant wear in the range of 200-750°C is surprising because the primary suspect which is oxidation is thermally activated. The oxide scale though expected to be thin at low temperatures, has to increase in thickness with temperature due to increased diffusion and reaction rates. The oxide scale also undergoes a change in morphology and composition which indicate a lower oxidation resistance as temperature increases. A preliminary characterization of the wear scar on the ball shows that the oxide inside the worn region is thinner than the oxide outside at 750°C. The amount of O within the wear scar is similar to levels observed on as deposited surface while the surface outside the wear scar shows oxidation and discolouration. The results suggest that oxidation inside worn region at high temperatures might be slower than the expected parabolic oxidation occurring outside the wear region. It is speculated that a double layer oxide is formed with TiO2 towards the surface and Al2O3 towards the nitride which is responsible for the lower wear at high temperature. This is supported by the fact that larger amount of Ti is found in the wear debris as temperature inceases. Superficial surface cracks appear at higher loads at temperatures as low as 600°C but they affect wear only above 800°C due to substrate softening. This shows that the coatings are still limited by the substrate softening temperature and could be used at higher temperatures.
Tribo-reaction in metals, nitrides and carbides can be brought about in the presence of O2 or water vapour. Tribochemical wear of SiN, SiC, TiN, TiAlN, alumina and most other ceramics at room temperature are found to depend on humidity[4]–[6]. But only tribo-oxidation due to O2 is found to operate at high temperatures[7], [8]. Notwithstanding, it is known that SiC and SiN are more resistant to attack from O2 above 800°C than from steam. Hence the role played by water vapour is found to be convoluted. Moreover, relative humidity is the frequently mentioned quantity with regard tribochemical wear at room temperatures. It should be noted that relative humidity is not a measure of chemical activity of water vapour. Rather the water vapour pressure which represents the chemical activity of water, is not given much importance in the earlier studies. In this study, the importance of humidity, water vapour pressure and temperature in influencing wear, is studied by performing controlled wear tests on TiN.
To explore the effect of temperature and water vapour pressure, TiN is tested varying temperature range of 28 °C to 90°C and water vapour pressure in the range of 3-35 mm-of-Hg. Wear tests are conducted keeping temperature constant with varying water vapour pressure and vice versa. The results show that, wear increased with humidity/vapour pressure at a fixed temperature but wear dropped drastically with increase in temperature at constant vapour pressure up to a critical temperature beyond which wear remained constant. This is one of the major unexpected findings since temperature is expected to increase wear volume. Also the critical temperature is found to shift to higher temperatures as water vapour pressures increased. It was suspected that capillary condensation was playing a role in the wear which was later verified. The whole wear behaviour is shown to be correlated with the amount of capillary condensed water. The large radius of curvature of the asperities on the polished coating surface and the smooth surface formed on the counter-face due to debris compaction form conditions favourable for capillary condensation. Any two hydrophilic surfaces which come in contact can form capillary condensation to occur at the cusps formed around the contact. However a threshold pore size of about 1nm existed below which condensation did not influence wear.
Another observation is that the water vapour did not affect wear significantly in the absence of condensation for TiN coatings. As temperatures increased condensation became unfavourable, but the high vapour pressure present showed no signs of wear enhancement. This is surprising and unexpected compared to earlier reports.[9], [10] On contrary tests in liquid water showed expected behaviour for tribochemical reaction i.e wear increased with temperature. The wear in liquid water is highest when compared studies in air at any given temperature. The X-ray electron emission spectroscopy (XPS) analysis is performed to understand the surface reactions. It appears that O2 forms a barrier oxide which protects the nitride from reacting with water vapour. However when condensation occurs or in water, the oxygen and water collude into forming softer hydroxide layer which is easily removed. Though chemically water and water vapour are same, they affect wear in TiN very differently.
Summarising the synopsis, exploration into high temperature wear of TiAlN reveals that it can handle oxidative wear upto 750°C showing constant wear over the temperature range of 200-750°C. Reduction in residual stresses and substrate softening may be responsible for higher wear at higher loads since the cracking is observed at 5N is absent at 3N. The substrate is expected to soften above 650°C but this does not necessarily affect wear until the load is increased or the temperature is sufficiently high. However TiAlN and TiN coatings showed susceptibility to tribo-corrosion in water and high humidity at room temperature. At high humidity, condensation of water leads to increase in wear. The dependence of wear on humidity is found to be because of capillary condensation. The negligible dependence of wear on humidity in the absence of condensation is ascribed to formation of oxide layer due to reaction with O2 and coating. The oxide barrier formed due to atmospheric O2 protects the coating from reacting with the water vapour. The oxide barrier on TiN forms faster indicating O2 reaction to be faster than the reaction with water vapour. In the presence of capillary condensation or water, O2 is depleted from contacting surfaces thus hindering the formation of the barrier oxide, increasing wear. As temperature increases the condensation becomes unfavourable and barrier oxide dominates the wear mechanism upto high temperatures which is dominated by oxidative wear.
| Identifer | oai:union.ndltd.org:IISc/oai:etd.iisc.ernet.in:2005/3648 |
| Date | January 2017 |
| Creators | Govindarajan, Sumanth |
| Contributors | Jayaram, Vikram |
| Source Sets | India Institute of Science |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
| Relation | G28475 |
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