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Remanufacturing and Advanced Machining
oxidation or wear, biocompatibility, wetting, adhesion, durability, catalytic activity,
and toughness (Quintino, 2014). In methods based on thermal approach, like plasma
spraying, fine molten or semi-molten particles are sprayed onto a surface. This technique offers coating thicknesses from several micrometers to over 100 μm using different materials, such as ceramics, plastics, alloys, and composites. Residual stress
can, however, negatively affect the stability and, hence, the durability of a coating
layer, which can be considered a major disadvantage of the thermal coating methods
(Rasouli et al., 2021). In atmospheric pressure plasma spraying, powders of sprayed
materials are introduced into a plasma torch and melted or partially melted powders
at high temperature are accelerated toward the substrate at a high speed. Plasma
spraying forms a coating with a lamellae structure and the ability to deposit almost
any metals and various combinations of materials. A very high temperature of the
arc core allows for use of coating materials with a high melting point such as ceramics, cermets, and refractory materials (Chu et al., 2002). As-obtained plasma-sprayed
ZrO 2 –8 wt% Y 2 O 3 and (Ba,Sr) Al 2 Si 2 O 8 (BSAS)-mullite coatings have been used
by NASA as thermal barrier coatings for superalloy components and environmental
barrier coatings for SiC/SiC ceramic matrix composite systems, respectively. Yttria
stabilized zirconia (YSZ) coating remains stable at high operating temperatures,
has a low thermal conductivity of 2 W/(m·K) for bulk and 1.5–2.05 W/(m·K) for
plasma-sprayed YSZ. Multilayer ceramic coatings that incorporate mullite, YSZ,
and ultra-high-temperature zirconium diboride ZrB 2 are suitable for various systems
of thermal protection in rocket exhaust cones, insulating tiles for space shuttles, as
well as engine components and ceramic coatings that are embedded into airplane
windshield glass (Morks et al., 2013).
Plasma immersion ion implantation (PIII) is also sometimes referred to as
plasma implantation, plasma ion implantation, or plasma-based ion implantation. In
this process, specimens are surrounded by a high-density plasma and pulse biased to
a high negative potential relative to the chamber wall. Ions generated in the overlying plasma are accelerated across the sheath formed around samples and implanted
into target surfaces. In PIII, gaseous plasma can be generated by DC or capacitively/
inductively coupled radio-frequency glow discharge (rfGD), while metallic plasma
can be generated separately or simultaneously by vacuum arc plasma sources. At
the same time, many elements can be introduced into the plasma (Chu et al., 2002).
Air or oxygen plasma has found its application in superior cleaning and activation, providing advantages over wet chemical and UV/ozone activation in terms of
energy input, safety, hazardous waste, corrosion, thermal load, processing time, and
versatility in handling a range of material surfaces. In comparison with oxygen or
air techniques, plasma activation contains a larger number of variables to be optimized, which further expands the potential for the process improvement (Rasouli
et al., 2021).
A cost-effective electrochemical procedure for surface treatment of steels employing plasma is plasma electrolytic oxidation (PEO). It can be mainly applied to valve
metals or their alloys including Ti, Al, Mg, and Zr, and recent reports also include
non-valve metals like Zn, Hf, Ta, and Nb, as well as diluted alkaline and environmental-friendly solutions containing phosphate, silicate, and aluminate anions. With
Remanufacturing and Advanced Machining
oxidation or wear, biocompatibility, wetting, adhesion, durability, catalytic activity,
and toughness (Quintino, 2014). In methods based on thermal approach, like plasma
spraying, fine molten or semi-molten particles are sprayed onto a surface. This technique offers coating thicknesses from several micrometers to over 100 μm using different materials, such as ceramics, plastics, alloys, and composites. Residual stress
can, however, negatively affect the stability and, hence, the durability of a coating
layer, which can be considered a major disadvantage of the thermal coating methods
(Rasouli et al., 2021). In atmospheric pressure plasma spraying, powders of sprayed
materials are introduced into a plasma torch and melted or partially melted powders
at high temperature are accelerated toward the substrate at a high speed. Plasma
spraying forms a coating with a lamellae structure and the ability to deposit almost
any metals and various combinations of materials. A very high temperature of the
arc core allows for use of coating materials with a high melting point such as ceramics, cermets, and refractory materials (Chu et al., 2002). As-obtained plasma-sprayed
ZrO 2 –8 wt% Y 2 O 3 and (Ba,Sr) Al 2 Si 2 O 8 (BSAS)-mullite coatings have been used
by NASA as thermal barrier coatings for superalloy components and environmental
barrier coatings for SiC/SiC ceramic matrix composite systems, respectively. Yttria
stabilized zirconia (YSZ) coating remains stable at high operating temperatures,
has a low thermal conductivity of 2 W/(m·K) for bulk and 1.5–2.05 W/(m·K) for
plasma-sprayed YSZ. Multilayer ceramic coatings that incorporate mullite, YSZ,
and ultra-high-temperature zirconium diboride ZrB 2 are suitable for various systems
of thermal protection in rocket exhaust cones, insulating tiles for space shuttles, as
well as engine components and ceramic coatings that are embedded into airplane
windshield glass (Morks et al., 2013).
Plasma immersion ion implantation (PIII) is also sometimes referred to as
plasma implantation, plasma ion implantation, or plasma-based ion implantation. In
this process, specimens are surrounded by a high-density plasma and pulse biased to
a high negative potential relative to the chamber wall. Ions generated in the overlying plasma are accelerated across the sheath formed around samples and implanted
into target surfaces. In PIII, gaseous plasma can be generated by DC or capacitively/
inductively coupled radio-frequency glow discharge (rfGD), while metallic plasma
can be generated separately or simultaneously by vacuum arc plasma sources. At
the same time, many elements can be introduced into the plasma (Chu et al., 2002).
Air or oxygen plasma has found its application in superior cleaning and activation, providing advantages over wet chemical and UV/ozone activation in terms of
energy input, safety, hazardous waste, corrosion, thermal load, processing time, and
versatility in handling a range of material surfaces. In comparison with oxygen or
air techniques, plasma activation contains a larger number of variables to be optimized, which further expands the potential for the process improvement (Rasouli
et al., 2021).
A cost-effective electrochemical procedure for surface treatment of steels employing plasma is plasma electrolytic oxidation (PEO). It can be mainly applied to valve
metals or their alloys including Ti, Al, Mg, and Zr, and recent reports also include
non-valve metals like Zn, Hf, Ta, and Nb, as well as diluted alkaline and environmental-friendly solutions containing phosphate, silicate, and aluminate anions. With