A Look at Ceramic Coatings
Coatings serve many functions. Some are designed largely to provide a desired appearance or effect on a substrate. Others are intended to be mainly protective in nature. In many applications, paints are expected to afford a combination of aesthetics and protection. These properties can be achieved using a wide range of materials. Common coatings in the automotive, architectural, and special purpose segments are based on polymer resins, or binders, that via various mechanisms undergo film formation once applied to a surface. Many industrial coatings are also polymeric in nature. There are cases, however, where extreme conditions (temperature, pressure, chemical/radiation exposure, etc.) are encountered during manufacturing. For these applications, ceramic coatings are often used.
Ceramic coatings, unlike coatings based on polymer resins, are inorganic materials. These non-metallic coatings have unique properties that cannot be achieved with organic coatings based on polymers, such as high heat, abrasion/wear, and chemical resistance. By choosing the right composition of elements, it is possible to create coatings with customized properties that meet the needs of a wide range of industrial applications.
Common ceramic coatings include alumina, titania, zirconia, alumina-magnesia, hafnia, silicon carbide, silicon nitride, and boron carbide, as well as oxides of many of these materials. The global ceramic coatings market is estimated by market research firm Market Research Future to expand at a compound annual growth rate of 7.6% between 2016 and 2023 to reach a value of $18.14 billion. This growth is being driven by increasing demand from the aerospace, defense, and automotive industries. Ceramic coatings are also used in the oil and gas, petrochemical, steel, plastics, textile, and other industries. Geographically, Asia-Pacific accounted for 38% of the ceramic coatings market by value in 2014, according to market research firm Markets and Markets.
By choosing the right composition of elements, it is possible to create coatings with customized properties that meet the needs of a wide range of industrial applications.
Ceramic coatings can be applied at thicknesses ranging from a few to several hundred microns. Common application methods include physical vapor deposition (PVD), chemical vapor deposition (CVD), thermal spraying, plasma spraying dipping, sol gel, micro-oxidation, packed diffusion, ionic beam surface treatment, and laser assisted techniques. Ceramic coatings are generally applied to metal surfaces to protect them from wear and tear, damage due to high temperatures, and/or corrosion and chemical attack in harsh environments.
The choice of a specific ceramic coatings depends on the substrate to be protected and the conditions against which protection must be provided. The properties of the base metal, including its metallurgy, hardness, toughness, and thermal stability, should be considered. The environment, including contact pressures, temperatures, whether abrasive wear is an issue, and if so, the nature of the wear, whether reduce friction is required, etc., are all important factors. Carbides tend to be harder while offering lower friction, oxides have low thermal conductivity, and nitrides exhibit high chemical resistance along with surface compatibility in motion.
For instance, thermal protection is needed in gas turbines and for piston crowns in diesel engines, while low friction coatings are often used to produce bearings that do not require the use of a separate lubricant, and corrosion-resistant ceramic coatings find use in the process industries. Alumina coatings tend to have low strength and fracture toughness but have high hardness and provide good electrical insulation, heat, and wear resistance. As a result, they are some of the most widely used ceramic coatings, finding use in many different applications, such as wear-resistant coatings for alumina parts, coatings for metal substrates that require temperature and corrosion resistance, and electrically insulating coatings. Zirconia, on the other hand, has high strength and fracture toughness combined with high hardness and good wear and corrosion resistance, as well as electrical insulation and low thermal conductivity. There are a number of different zirconia alloys that offer a range of property combinations through the addition of various oxides (e.g., magnesium and calcium oxides) and rare-earth elements (e.g., cerium oxide).
Silicon carbide is a very hard material with a high thermal conductivity, leading to its use in bearing and rotary seal applications. The properties of silicon nitride—electrical insulation, resistance to molten metals, low thermal conductivity, good thermal shock resistance—have led to its use in RF heating applications. Because of its high strength, silicon nitride is also widely used in the automotive and machine tool industries for bearing and wear parts exposed to abrasive conditions. Boron carbide is a very hard material that provides excellent resistance to abrasive wear. It is also used as a thermal-neutron absorber.