Thermal spray coatings for soft bearing surfaces allow the embedding of abrasive particles and permit deformation to accommodate some misalignment of the bearing surfaces. These surfaces require adequate lubrication and should be low in cost as they wear in preference to the mating surface which are usually very much harder. Some of these coatings are quite porous with the advantage that they act as reservoirs for lubricants. Thermal spray coatings for soft bearing surfaces commonly used include aluminum bronze, phosphor bronze, white metal or babbitt, and aluminum bronze-polymer composites.
Thermal spray coatings for hard bearing surfaces are hard and have high wear resistance. Hard bearing materials are used where the embedding of abrasive particles and self-alignment are not required and where lubrication may be marginal. The inherent nature of thermal spray coatings seems to provide additional benefits over comparable wrought or cast materials due to the porosity acting as a lubricant reservoir and the composite nature of included oxides and amorphous phases increasing wear resistance. Some coatings show relatively low macrohardness compared to wrought or cast materials, but very often show improved wear resistance. Thermal spray coatings used for hard bearing surfaces typically include cermet coatings like tungsten carbide-cobalt and chromium carbide-nickel chromium, oxide ceramics like chromium oxide and alumina, molybdenum, and various hard alloys of iron, nickel, chromium or cobalt.
Abrasion Resistant Coatings
Ideally, the materials for thermal spray coatings for resistance to abrasion should have a hardness that is in excess of that of the mating surface or abrasive particles. The coatings commonly used are cermet coatings like tungsten carbide-cobalt, chromium carbide-nickel chromium (particularly for high temperatures above 540 °C), oxide ceramics like chromium oxide and alumina, fused self fluxing alloys (NiCrSiB), and various hard alloys of iron, nickel, chromium or cobalt. h3>Wear (scuff/fretting) Resistant Coatings Coatings resistant to wear caused by repeated sliding, rolling, impacting or vibration are generally coatings with good toughness and low residual tensile stress. The thermal spray coatings for resistance to fretting and surface fatigue commonly used include cermet coatings like tungsten carbide-cobalt, chromium carbide/nickel chromium (particularly for high temperatures above 540 °C), fused self fluxing alloys, aluminum bronze, copper nickel indium, and various alloys of iron, nickel, chromium or cobalt.
Erosion Resistant Coatings
The selection of coating for erosive wear is dependent on the severity and type of erosion. For solid impingement erosion at a shallow angle of attack where the wear is similar to that of abrasion, high hardness coatings are required. For solid impingement angles near 90°, coating toughness becomes more important. For cavitation and liquid impingement generally, a coating with good surface fatigue resistance is needed. Thermal spray coatings for resistance to erosion commonly used include cermet coatings like tungsten carbide-cobalt, chromium carbide-nickel chromium (particularly for high temperatures above 540 °C), fused self fluxing alloys, non-ferrous alloys, aluminum bronze, monel, oxide ceramics like chromium oxide and alumina, and various alloys of iron, nickel, chromium or cobalt.
PTFE polymer type materials have extremely low coefficient of friction and are "non-sticking" to most materials. These particular properties are very useful, but these materials have very low strength and very poor wear resistance. Combination coatings, to provide the mechanical support, keying for the polymer, and wear resistance, offer an extremely effective compromise.
Corrosion Resistant Coatings
Thermal spray coatings are widely used in preventing corrosion of many materials, with very often additional benefits of properties such as wear resistance. Thermal spray coatings for corrosion protection fall into three main groups:
Anodic coatings for the protection of iron and steel substrates are almost entirely limited to zinc and aluminum coatings or their alloys. Where coatings anodic to the substrate are applied, the corrosion protection is referred to as cathodic protection or sacrificial protection. The substrate is made to be the cathode and the coating the sacrificial corroding anode. The metallizing process is an excellent means of protecting iron and steel from corrosion to almost any desired degree, from long life coatings to inexpensive coatings which are competitive with organic coatings such as paint. Heavy coatings of zinc or aluminum can be applied to meet the most severe corrosion conditions and give 15 to 50 years life without any further maintenance. Aluminum has been found to be the most effective metal for protection of steel in offshore structures.
Cathodic coatings comprise a metal coating which is cathodic with respect to the substrate. A stainless steel or nickel alloy coating would be cathodic to a steel base. Cathodic coatings can provide excellent corrosion protection. There is a very wide choice particularly for steel base materials ranging from stainless steel to more exotic materials like tantalum to cater for the more extreme corrosive environments. However, a limitation of such coatings is that they must provide a complete barrier to the substrate from the environment. If the substrate is exposed to the corrosive environment, the substrate will become the anode and corrosion will be dramatically accelerated resulting in spalling of the coating. Generally, sealing of these coatings is always recommended. Processes, which provide the densest coatings, are preferred (HVOF, plasma and fused coatings). Thick coatings will provide better protection than thin coatings.
Neutral materials such as alumina or chromium oxide ceramics provide excellent corrosion resistance to most corrosive environments by exclusion of the environment from the substrate. Generally, a neutral material will not accelerate the corrosion of the substrate even if the coating is somewhat permeable, but any corrosion of the substrate interface with the coating should be avoided to prevent coating separation. Again, sealing of the coatings is recommended. The densest and thickest plasma sprayed coatings are recommended. When stainless steel type substrate materials are used where the exclusion of oxygen can cause crevice corrosion, nickel chromium bond coats are required.
Tool and Die Coatings
Tooling and die costs in metalworking operations contribute significantly to total production costs. Despite the high investment, wear leads to early failure of metalworking dies. Thermal spray deposition of wear resistant materials onto the parts of a die most prone to wear economically extends die life. An example is thermal spray deposition and high heat flux infrared post-treatment of chrome carbide coatings. Other coating materials that extend die life include high temperature metallic materials that have known wear resistance and good fatigue life; oxidation resistant materials known for their extreme levels of wear resistance; and oxidation resistant materials which provide protection in thermal environments where wear and oxidation are limiting factors.
Thermal Spray for Resurfacing
Thermal spray is an established industrial method for the surfacing and resurfacing of metal parts. The benefits are typically lower cost, improved engineering performance, and/or increased component life. In addition to original equipment applications, thermal spray coatings are used to repair parts worn and damaged in service, and restore dimensions to machined parts. Thermal spray coatings are used to restore the dimensions of components that have been worn or corroded, such as printing rolls and undersized bearings. Although the thermal spray coating does not add any strength to the component, it is a quick and economical way to restore the dimensions of parts. Subsequent grinding operations are often needed to smooth the coating's surface and to bring the final dimensions into their appropriate tolerances. Thermal spray coatings for dimensional restoration are being used in every manufacturing industry.
The aerospace, automotive, and electronic packaging industries are the largest uses of ceramic dielectric coatings. Dielectric coatings are either pure aluminum oxide or a spinel. In either case a very high density coating can be created that is capable of withstanding thousands of volts depending on the coating thickness.
Thermal sprayed release coatings use a matrix, which is impregnated with a release agent of either Teflon or Silicone. Release coatings are used to provide a component with anti-stick characteristics as well as wear resistance. Components utilizing thermal sprayed release coatings are typically used in the manufacturing of plastics, adhesives, rubber, or food products.
Traction coatings are used on rolls in the printing and papermaking industry to grab and feed paper. Because the traction of the coating depends substantially on the degree of its surface roughness, nearly any material can be used to create a traction coating. However, in most applications where a traction coating is required there is also a great amount of wear present and, therefore, the most common traction coating materials are carbides, stainless steels, and nickel alloys.