1. Surface Engineering By Israa Faisal University of Al-Qadisiyah
College of Engineering
Material Engineering Department

2. Thermal evaporation
Thermal evaporation is the oldest and probably the most widely used PVD technique. It accounts for the major proportion of both the equipment in use and the area coated. Thermal evaporation occurs in a hard vacuum of 0.1 to 10 mPa, at which pressures the mean free path of a gas atom, that is, the average distance the atom travels before colliding with another atom, is greater than the chamber dimensions. An atom evaporating from a source travels in a straight line; thus the process is line-of-sight limited, and coating around corners or reentrant angles is not possible without substrate manipulation.

3. Sputter coating
Sputter coating is a vacuum process that involves the use of ions from a gas-generated plasma to dislodge coating atoms or molecules from a target made of the material that will become the coating. The plasma is established between the target and the substrate by the application of a direct- current potential or an alternating potential (radio frequency). An inert gas is introduced into the chamber to form the glow discharge plasma between the electrodes. The materials that can be sputter coated are pure metals, alloys, inorganic compounds, and some polymeric materials. A major restriction to be considered for the substrate material is the temperature of the process, which can range from 260 to 540 0C ( F). Sputtering is often used for depositing compounds and materials that are difficult to coat by thermal evaporation techniques.

4. Ion plating
Ion plating is a vacuum coating process in which a portion of the coating species impinges on the substrate in ionic form. The process is a hybrid of the thermal evaporation process and sputtering with the evaporation rate being maintained at a higher rate than the atoms that can be sputtered from the substrate. Some evaporant atoms pass through the plasma in atomic form, while some atoms collide with electrons from the substrate and become ions. They impinge on the substrate in ionic form, pick up electrons, and return to the atomic state, forming the coating.

5. Thermal Spray Coatings
Thermal spraying is a generic term for a group of processes that apply a consumable in the form of a spray of finely divided molten or semimolten droplets to produce a coating. The characteristics that distinguish thermal spray processes are indicated as follows:
Substrate adhesion, or bond strength, is dependent on the materials and their properties and generally is characterized as a mechanical bond between the coating and the substrate.
Spray deposits can be applied in thinner layers than welded coating, but thick deposits are also possible.
Provided there is a stable phase, almost all material compositions can be deposited, including metals, cermets, ceramics, and plastics.
Thermal spray processes are usually used on cold substrates, preventing distortion, dilution, or metallurgical degradation of the substrate.
Thermal spray processes are line-of-sight limited, but the spray plume often can be manipulated for complete coverage of the substrate.

6. Thermal Spray Coatings
Thermal spray processes can be classified into two categories, arc processes and gas combustion processes, depending on the means of achieving the heat for melting of the consumable material during the spraying operation.
In the lower-energy electric arc (wire arc) spray process, heating and melting occur when two electrically opposed charged wires, comprising the spray material, are fed together to produce a controlled arc at the intersection.
The molten material on the wire tips is atomized and propelled onto the substrate by a stream of gas (usually air) from a high-pressure gas jet. The highest spray rates are obtained with this process, allowing for cost-effective spraying of aluminum and zinc for the marine industry. In the higher-energy plasma arc spray process, injected gas is heated in an electric arc and converted into a high-temperature plasma that propels the coating powder onto the substrate at very high velocities. This process can take place in air with air plasma spraying (APS), or in a vacuum with vacuum plasma spray (VPS).

8. Thermal Spray Coatings
For gas combustion processes, the lower-energy flame spray process uses oxyfuel combustible gas as a heat source to melt the coating material, which may be in the form of rod, wire, or powder. In the higher-energy, high-velocity oxyfuel combustion spray (HVOF) technique, internal combustion of oxygen and fuel gas occurs to produce a high-velocity plumecapable of accelerating powders at supersonic speeds and lower temperatures than the plasma processes. Continuous combustion occurs in most commercial processes, whereas the proprietary detonation gun (D-gun) process uses a spark discharge to propel powder in a repeated operating cycle to produce a continuous deposit.

9. Properties of Thermal Spray Coatings
The variations in oxide content and porosity, as well as the chemical composition of the coating, greatly affect the properties of the deposit and, in the case of corrosion, the underlying substrate. The splat morphology and, more importantly, the splat/splat and splat/substrate interface are critical to properties such as bond strength, wear, erosion, and corrosion. The mechanical properties of thermal spray coatings are not well documented except for their hardness and bond strength. Generally, the wear resistance of coatings increases with their density and cohesive strength, so that HVOF coatings provide the best wear resistance in contrast to plasma spray coatings. Carbide cermets were found to be good for both wear and erosion environments, and the optimal amount of hard phase (oxide and carbide) has been determined for erosion resistance.

10. Thermoreactive Deposition/Diffusion Process
(TRD) is a method of coating steels with a hard, wear-resistant layer of carbides, nitrides, or carbonitrides. In the TRD process, the carbon and nitrogen in the steel substrate diffuse into a deposited layer with a carbide-forming or nitride forming element such as vanadium, niobium, tantalum, chromium, molybdenum, or tungsten. The diffused carbon or nitrogen reacts with the carbide- and nitride-forming elements in the deposited coating so as to form a dense and metallurgically bonded carbide or nitride coating at the substrate surface.

11. Thermoreactive Deposition/Diffusion Process
The TRD process is unlike conventional case-hardening methods, where the specific elements (carbon and nitrogen) in a treating agent diffuse into the substrate for hardening. Unlike conventional diffusion methods, the TRD method also results in an intentional buildup of a coating at the substrate surface. These TRD coatings, which have thicknesses of about 5 to 15 μm ( mil), have applications similar to those of coatings produced by CVD or PVD.
The most important properties associated with TRD coatings are high hardness and wear resistance. Figures 1 and 2 compare the surface hardness and wear properties of TRD coatings with various other surface hardening processes.

12. Thermoreactive Deposition/Diffusion Process

13. Thermoreactive Deposition/Diffusion Process