The particle sizes of the raw materials used for coatings are characterized using a laser particle size analyzer. Microhardness measurement is done on the smooth cross-section of the samples (in the optically distinct phases of the coating), using the Leitz Micro-Hardness Tester. To study the suitability of coatings for wear resistance applications, the wear properties of these coatings are studied.
Factors to consider include: a) the properties of the coating material itself - its melting point, hardness, vapor pressure, density and coefficient of thermal expansion. Chemical interaction of the coating-forming element(s) with the substrate by diffusion is involved in this category. The production rate of the process is very high and the coating adhesion is also sufficient.
Objectives of the present piece of investigation
Surface engineering' is the name of the discipline - surface modification is the philosophy behind it. If a thin TiN coating is applied to the WC-Co insert, its performance is significantly increased . Thus, with the process of surface modification, we combine two (or more) materials using the appropriate method and take advantage of the properties of both .
Techniques of surface modification
Surface modification is a relatively new term that has emerged in the last two decades to describe interdisciplinary activities aimed at tailoring the surface properties of engineering materials. In fact, the cutting tool is exposed to a high degree of abrasion during operation, and TiN is more capable of combating wear. On the other hand, TiN is extremely brittle, but the relatively strong core of the WC-Co composite protects it from fracture.
The high melting temperature is achieved chemically (by combustion) or electrically (in an arc); these melting methods also accelerate the molten particles to the target substrate where the material solidifies and forms a deposit. The spraying effect is achieved by the rapid expansion of the combustion gases (which transfer momentum to the molten droplets) or by the separate supply of compressed air. The processes available for thermal spraying have been developed specifically for a specific purpose and fall into two categories – high and low energy processes.
Porosity was found to increase and coating thickness (hence, deposition efficiency) to decrease with increasing offset distance. A greater proportion of unmelted particles goes into the coating due to the increase in the distance between the burner and the base. It was found that the injection angle also affects the cohesion and adhesion strength of the coatings .
Industrial applications of plasma spraying…
For example, in a given situation, if the size of the powder is too small, it may evaporate. The residence time of the powders in the plasma jet will vary with the injection angle for a given carrier gas flow rate. For example, for melting materials with a high melting point, a long residence time and thus oblique injection may prove beneficial.
The phase transformation during freezing of plasma-sprayed alumina droplets has been studied in detail [63,64]. The wear rate of the nanostructured coating was lower than that of the conventional coating . An erosion mechanism model is advanced based on the aforementioned erosion wear behavior as shown in Figure 2.10.
Characterizations of the coatings were done and the tribological performances of the coating were evaluated. From the figure, it is clear that the adhesion strength varies with the operating power of the plasma torch. The less erosion wear rate is one of the main requirements of the coatings developed by plasma spraying.
Figure 4.8 Relative effect of main factors on erosion rate of coatings produced at 11 kW. Figure 4.9 Relative effect of main factors on erosion rate of coatings made at 18kW. It further shows that this dependence is also influenced by the nature of the coating material.
The adhesion of the coating interface depends on the morphology of the coating and the bond between the particles of the sprayed powders. The coating substrate interface plays the most important role in coating adhesion. The surface morphology of the coating cannot predict the internal structures (layer deposition) and their importance/acceptability.
It is observed that the amount of porosity is more in the case of coatings made at lower (11kW) and higher (21kW) power levels. Microstructure, adhesion, microhardness, wear resistance of the plasma sprayed alumina and alumina – titanium oxide coatings.” , Thin solid film.
Wear occurs as a natural consequence when two surfaces with a relative movement interact with each other. Wear can be defined as the progressive loss of material from contact surfaces in relative motion. Researchers have developed different theories of wear, taking into account the physico-mechanical properties of the materials and the physical conditions (e.g. the resistance of the rubbing body and the state of stress at the contact area).
Wear of metals is probably the most important yet least understood aspect of tribology. It is certainly the youngest of the three subjects, friction, lubrication and wear, to attract scientific attention, although its practical significance has been recognized over the centuries. Wear is not an intrinsic material property, but characteristics of the engineering system that depend on load, speed, temperature, hardness, presence of foreign material and the environmental condition .
This may be due to surface damage or removal of material from one or both of two solid surfaces in a sliding, rolling or impacting motion relative to each other. During relative movement, material on contact surface can be removed from a surface, can lead to the transfer to the mating surface, or can break off as a wear particle. Wear of metals depends on many variables, so wear research programs must be systematically planned.
The wear map proposed by Lim and Ashby  is very useful in this regard to understand the wear mechanism in sliding wear, with or without lubrication.
Types of wear
- Abrasive wear
- Adhesive wear
- Erosive wear
- Surface fatigue wear
- Corrosive wear
Adhesive wear can be defined as wear due to local bonding between solid surfaces in contact, leading to material transfer between the two surfaces or loss from both surfaces. Erosive wear can be defined as the process of metal removal due to impact of solid particles on a surface. Erosion is caused by a gas or liquid, which may or may not carry entrained solid particles, impinging on a surface.
When the collision angle is small, the wear produced is very analogous to abrasion. When the impact angle is perpendicular to the surface, material is displaced by plastic flow or is displaced by brittle failure. Repeated loading causes the generation of micro cracks, usually below the surface, at the site of a pre-existing point of weakness.
When the crack reaches the critical size, it changes its direction to emerge at the surface, thus releasing flat plate-like particles during use. The number of stress cycles required to cause such failure decreases as the corresponding magnitude of stress increases. Most metals are thermodynamically unstable in air and react with oxygen to form an oxide, which usually develops layers or scales on the surface of metal or alloys when their interfacial bonds are poor.
Corrosion Wear is the gradual erosion or deterioration of protected metal surfaces as a result of atmospheric influences, acids, gases, alkalis, etc.
Symptoms of wear
A typical model illustrating the rate of erosion as a function of particle size and velocity upon impact is shown in Figure 2.9. From the fact that increasing the speed or size of the particles leads to larger or deeper indentations, as shown schematically in Figure 2.7, the deviations of the k2 and k3 values from the theoretical ones (k2=2, k3= 0) indicate real impact effects. particle velocity and diameter, which are related to the relative aggressiveness of the indentation.
The larger or deeper the indentation, the greater the amount of material removed from the edge of the indentation. Delamination Presence of subsurface cracks parallel to the surface with semi-loose or loose flakes.
Recent trends in metal wear research
Wear resistant coatings
This material was also tested in lubricated conditions using solutions of inorganic salts (NaCl, NaNO3, Na3PO4) as lubricants and also at high temperature.
There are several advantages of aluminum oxide as a structural material, e.g. accessibility, hardness, high melting point, wear resistance, etc. Some uses of aluminum oxide are in bearings, valves, pump seals, pistons, engine components, rocket nozzles, shields for guided missiles, vacuum tube casings, integrated circuits, etc. The properties of aluminum oxide can be additionally supplemented by reinforcement from particles (TiO2, TiC) or fibers (SiC) .
The slip coating behavior of monolithic aluminum and reinforced with SiC whiskers has been studied . Monolithic aluminum has a brittle response to sliding wear, while the worn surface of the composite reveals signs of plastic deformation along with fracture. The sliding wear behavior of plasma sprayed aluminum against AISI-D2 steel under different speed loading conditions is reported.
Plastic flow leads to an increase in the effective contact area and a corresponding decrease in the normal stress even as the normal strain increases. At low speeds, the bumps move towards each other and deform in the process. As the speed increases, bumps are subjected to heavy impacts and tend to break from the root, resulting in a larger amount of debris.
At very high speed, the temperature rise due to friction becomes high enough to soften the bumps and thus protect them from breaking.
The deposition efficiency is defined as the ratio of the weight of the coating deposited on the substrate to the weight of the spent raw material. The thickness of ash-ilmenite coatings on different substrates was measured on smooth cross-sections of the samples, using an optical microscope. The porosity of the coatings was measured by placing smooth cross-sections of the coating sample under a microscope (Neomate) equipped with a CCD camera (JVC, TK 870E).
The schematic diagram of the sand blast type erosion test rig is shown in Fig.3.6. Coating deposition efficiency is defined as the ratio of the weight of coating deposited on the substrate to the weight of the raw material consumed. However, the plasma operating power above which the efficiency decreases depends on the chemical nature of the feed material i.e.
To ensure the ash-ilmenite coating on different substrates, the coating thickness was measured on smooth cross-sections of the samples, using an optical microscope. From the above, the erosion behavior of the ash-ilmen coating deposited at 15 kW eroded by sand is like the behavior of brittle material. Smooth cross-sections of the samples were examined under SEM and are shown in Fig.
The operating power level of the plasma torch affects the coating adhesion strength, deposition efficiency, coating thickness and coating hardness to a great extent.