Thermal Spray Ceramic Coating Technology and Application

The use of thermal spraying technology, especially plasma spraying technology, to prepare ceramic coatings on metal substrates can organically combine the characteristics of ceramic materials with those of metal materials to obtain composite material structures and products.

1.

According to the definition of the international standard GB/T18719-2002, thermal spraying technology is a method of using a heat source to heat the spraying material to only melt or half-melting state, and spraying and depositing it on the pretreated substrate at a certain speed to form a coating, giving the substrate a method of forming a coating.

The main factors affecting the performance of thermal spray coatings are the molten state (temperature) and flight speed (the kinetic energy of particles) of the raw materials in the spraying flame

↓↓Advantages and existing problems of different spraying processes

There are many technological methods of thermal spraying technology, but no matter what kind of technological method, the principle and structure of the coating formed during the spraying process are basically the same

1.

When the spraying material is a wire (rod) material, during the spraying process, the end of the wire continuously enters the high-temperature area of ​​the heat source to be heated and melted to form droplets; when the spraying material is powder, the powder material directly enters the high-temperature area of ​​the heat source.

2.

The wire (rod) material is heated and melted to form droplets during the spraying process.

Thermal spray technology coating formation principle

3.
Flight stage

Particles in a heated melted or semi-melted state are accelerated to fly under the action of an external compressed airflow or the heat source's own airflow power
.
During the particle flight, the spray particles are first accelerated and decelerated as the flight distance increases
.

4.
Collision deposition stage

When the sprayed particles with a certain temperature and speed contact the base material, they impact the surface of the base material with a certain kinetic energy, resulting in a strong collision
.
At the moment of collision with the base material, the kinetic energy of the sprayed particles is converted into heat energy and transferred to the base material, resulting in deformation on the uneven surface of the base material
.
Due to the effect of heat transfer, the deformed particles rapidly condense with volume shrinkage.
Most of the particles are firmly bonded to the surface of the matrix material in a flat shape, while another small part bounces off the surface of the matrix after collision
.
With the continuous impact of the sprayed particle beam on the surface of the substrate, the collision-deformation-condensation-shrinkage-filling proceeds continuously
.
The deformed particles are bonded together on the surface of the base material in a staggered and superimposed manner between particles, and finally deposited to form a coating
.

Schematic diagram of the process of coating formation

Second, the application of thermal spray ceramic coating

Ceramics are crystalline or amorphous compounds composed of metallic and non-metallic elements
.
Together with metal materials and polymer materials, it constitutes the three pillars of solid-state engineering materials
.
Modern materials have classified cermets and other inorganic non-metallic materials into the category of ceramics, becoming a large family of materials with many varieties and functions
.
Generally speaking, any material that can be in a molten state or plastic state by heating can be used as a coating material for thermal spraying
.
In addition to metallic materials, ceramics can also be used for thermally sprayed anti-corrosion coatings
.
The ceramic materials commonly used in thermal spraying are mainly oxide ceramic materials such as Al2O3, TiO2, Cr2O3, ZrO2, and non-oxide ceramics such as carbides are usually prepared into metal matrix ceramic composite materials by using metal alloys as binders
.
Due to the brittleness of nitride and its poor oxidation resistance, it is rarely used
.

↓↓Common thermal spray materials and their characteristics and applications

Thermal sprayed ceramic coatings can be divided into several categories according to application fields: wear-resistant ceramic coatings, corrosion-resistant ceramic coatings, thermal barrier ceramic coatings, bioceramic coatings and piezoelectric ceramic coatings
.

1.
Wear-resistant coating

Statistics show that the loss caused by wear and corrosion accounts for about 3%-5% of the country's GDP in developed countries, while it is as high as 10% in developing countries, which causes the performance of parts to decline and eventually fail during processing and production
.
Thermal spray technology has been used as the most important method for effective wear reduction and corrosion protection technology
.

The energy and material losses caused by friction and wear originate from the material surface.
Therefore, various surface engineering technologies have become an important method to improve the anti-friction and wear resistance of components and materials
.
Thermal spray technology is one of the widely used surface engineering technologies
.
At present, coatings prepared by thermal spraying technology have been widely used in mechanical equipment
.
The wear resistance of thermal spray coatings mainly depends on the composition, phase composition, particle size and content of the coating, as well as the bonding strength, hardness, porosity, particle size and other factors of the coating
.
Coatings with high bonding strength, high hardness and low porosity have good erosion and wear resistance
.
At present, the commonly used wear-resistant coatings are: Al2O3 layer, Al2O3+TiO2 coating, Cr2O3 coating and WC-Co coating
.

↓↓Application fields of different thermal sprayed wear-resistant coatings

Only by selecting the appropriate coating material and the corresponding thermal spraying process can the anti-friction and wear-resistant coating suitable for the working conditions be prepared
.
Based on the rapid development of industry and technology, the working conditions of mechanical equipment are getting higher and higher, and it is necessary to develop anti-friction and wear-resistant coatings with better performance
.
On the one hand, starting from the components of the coating material, a new material system with high strength, hardness or self-lubricating function can be designed, such as adding a self-lubricating effect to a hard ceramic coating or metal-ceramic coating The lubricating phase (such as graphite, molybdenum disulfide, polymer materials, etc.
) can improve the antifriction performance of the coating
.
At present, most studies only propose adding lubricating phase to improve the anti-friction and wear-resisting properties of coatings, but do not thoroughly discuss conditions such as the optimal raw material ratio
.
On the other hand, process parameters are one of the important factors that affect the quality of thermal spray coatings.
We can start with optimizing the thermal spray process to study the best process parameters for different systems of coatings to lay the foundation for subsequent research work
.

2.
Corrosion-resistant ceramic coating

Steel used in harsh environments (such as marine salt spray environment, etc.
) is prone to corrosion, wear and loss of parts accuracy
.
The traditional surface treatment process carried out is mainly electroplating hard chromium, that is, hexavalent chromate is deposited on the cathode to form a wear-resistant and corrosion-resistant coating with high hardness, which is characterized by simple process and low cost
.
However, the electroplating process of chromium will cause serious environmental pollution, which is restricted in many industrial fields
.
In addition, the hydrogen embrittlement of the substrate will also occur during the process of chrome electroplating, which significantly reduces the mechanical properties of the substrate.
Therefore, it is urgent to find an effective alternative process
.
Thermal spray coatings have been used in the field of anti-corrosion because of their high efficiency, environmental protection and good chemical stability
.

According to the types of anti-corrosion coating materials, there are mainly aluminum, zinc and its alloy materials, nickel-based alloy materials, stainless steel materials and composite ceramic materials
.

3.
Thermal barrier ceramic coating

Ceramic thermal barrier coatings (TBCs) are often used on the surface of aero-engine turbine blades to separate the high-temperature components of the jet engine from the high-temperature gas to improve the service conditions of the high-temperature components.
On the one hand, it can improve the service life of the turbine blades, on the other hand can save fuel
.
In order for the ceramic thermal barrier coating to be well combined with the metal matrix to have better thermal insulation properties, the ceramic thermal barrier coating must have the following properties: ① lower thermal conductivity; ② thermal expansion coefficient matching the metal matrix; ③ Good phase stability is maintained at high temperatures
.
ZrO2-based ceramic materials with low thermal conductivity are usually selected as the material of the thermal barrier coating working layer
.

The main preparation technologies of ceramic thermal barrier coatings are plasma spraying, electron beam physical vapor deposition, laser cladding, high-speed flame spraying and explosive spraying, etc.
The two most commonly used preparation methods are plasma spraying (PlasmaSpray, referred to as PS) and electronic Beam physical vapor deposition (ElectronBeam-Physical VaporDeposition, referred to as EB-PVD)
.
The structure of plasma sprayed coating is characterized by a large number of pores and micro-cracks, which can relieve thermal stress and improve the thermal fatigue life of the coating
.

4.
Bioceramic coating

Titanium and its alloys, cobalt-chromium-molybdenum alloys, and stainless steel are all commonly used clinically used metal bone implant materials
.
The metal materials used clinically have no biological activity and need to be improved in combination with bone tissue
.
Surface modification of metal implants is a necessary way to improve their biological properties
.

Thermal spraying technology is one of the commonly used surface modification methods.
The biomedical coatings prepared by it mainly include metal oxide coatings (Al2O3, ZrO2, TiO2, etc.
), bioactive hydroxyapatite (HA) coatings,
etc.
Titanium and hydroxyapatite coatings have been widely used in clinical practice
.
In recent years, the research on calcium silicate bioceramic coatings has also received attention
.

5.
Piezoelectric ceramic coating

Piezoelectric ceramics are a class of ferroelectrics with excellent piezoelectricity, and are information-functional ceramic materials that convert mechanical properties and electrical energy into each other
.
In addition to the piezoelectric effect, piezoelectric ceramics also have dielectric and ferroelectric properties, and have been widely used in various fields such as medical imaging, acoustic sensors, acoustic transducers, ultrasonic motors, and display devices
.
In recent years, the preparation of piezoelectric ceramic coatings by spraying technology is gradually attracting people's attention
.

Reference source:

1.
Thermal spraying technology and application, edited by Wu Zijian; Machinery Industry Press
.

2.
Research progress of thermal spraying ceramic coatings; Liu Yingkai①, Yan Dianran②, Lu Xuecheng③, Meng Fanai①, Yao Junqing①; ①Boshen Tools Co.
, Ltd.
, ②Hebei University of Technology, ③Military Transportation College
.

3.
Research progress on the preparation of wear-resistant coatings by thermal spraying; Dang Zhe, Gao Dongqiang; School of Mechanical and Electrical Engineering, Shaanxi University of Science and Technology
.

4.
Research progress of zirconia-based ceramic thermal barrier coatings; Zhang Wei; Quality Control Section of Pailigu (Dalian) Industry Co.
,
Ltd.