High density and adhesion coating process and coatings formed thereby
A process of applying a coating to a substrate is provided. A layer of a metallic or ceramic material is applied on the substrate by a thermal spray process. The layer is then subject to plasma electrolytic oxidation (PEO) processing to densify the layer such that a porosity of the layer after PEO processing is lower than a porosity of the layer before PEO processing, or modify the layer by incorporating additional ceramic phases from the electrolyte bath into the densified layer, or improve the adhesion of the thermal spray layer. A base layer may be applied to the substrate with PEO processing before the above referenced layer is applied and/or a ceramic topcoat may be applied to the above referenced layer by a thermal spray process.
1 . A process of applying a coating to a substrate of electrical conductor, comprising the steps of:
applying a ceramic layer of ceramic material on the substrate by a thermal spray process selected from the group consisting of wire arc spraying, flame spraying, plasma spraying, and high velocity oxy-fuel spraying (HVOF); and
subjecting the ceramic layer produced by the thermal spray process to plasma electrolytic oxidation (PEO) processing in a bath of an electrolyte solution to densify the ceramic layer produced by the thermal spray process throughout a full thickness of the ceramic layer and to improve adhesion of the ceramic layer to the substrate, wherein a porosity of the ceramic layer after the PEO processing is less than 4%.
2 . A process of applying a coating to an electrically conductive substrate, comprising the steps of:
applying a base oxide layer on a surface of the substrate with a first plasma electrolytic oxidation (PEO) processing in a bath of an electrolyte solution;
applying a ceramic layer of ceramic material on the base oxide layer by a thermal spray process selected from the group consisting of wire arc spraying, flame spraying, plasma spraying, and high velocity oxy-fuel spraying (HVOF); and
after the thermal spray process, subjecting the ceramic layer produced by the thermal spray process to second plasma electrolytic oxidation (PEO) processing in a bath of an electrolyte solution to densify the ceramic layer produced by the thermal spray process throughout a full thickness of the ceramic layer, wherein the porosity of the ceramic layer after the second PEO processing is less than 4%.
3 . The process according to claim 1 , further comprising a step of applying a ceramic topcoat by a thermal spray process after the PEO processing.
4 . The process according to claim 1 , wherein, during PEO processing, the electrolyte solution includes an additive that modifies the ceramic layer, wherein the additive is selected from a metal oxide, a metal silicate, and a metal phosphate.
5 . The process according to claim 1 , wherein the thermal spray process is a high velocity oxy-fuel (HVOF) or plasma spray thermal spray process.
6 . The process according to claim 1 , wherein, during the PEO processing, a bipolar waveform with an anodic voltage in the range 400-700V and a cathodic voltage in the range 50-300V is used.
7 . The process according to claim 1 , wherein the oxide content in the layer after PEO processing is 10 to 70 vol. %.
8 . The process according to claim 1 , wherein the hardness of the layer after PEO processing is HV0.3 600-1800.
9 . The process according to claim 1 , wherein the substrate is made of aluminum, magnesium, titanium, yttrium, zinc, zirconium, niobium, cerium, tantalum, or alloys thereof.
10 . The process according to claim 1 , wherein the ceramic layer comprises a compound including at least one of aluminum, magnesium, titanium, yttrium, zinc, zirconium, niobium, cerium, and tantalum.
11 . The process according to according to claim 1 , wherein, during the PEO processing, the electrolyte solution has a pH of 10-14.
12 . The process according to claim 1 , wherein the ceramic layer after the PEO processing comprises at least one of aluminum oxide, phases of silicate, or phases of phosphate.
13 . The process according to claim 1 , wherein the ceramic layer after the PEO processing has a surface roughness Ra of less than 2 microns.
14 . The process according to claim 1 , wherein a thickness of the ceramic layer is 5 to 100 μm.
15 . The process according to claim 1 , wherein the substrate is made of a metal, a metal matrix composite, or a conductive ceramic composite.
16 . The process according to claim 1 , wherein a base ceramic layer is formed on a surface of the substrate to improve adhesion of the ceramic layer to the substrate.
17 . The process according to claim 16 , wherein the base ceramic layer is produced as a result of said PEO processing of the ceramic layer.
18 . The process according to claim 1 , wherein, during said PEO processing of the ceramic layer, a ceramic layer produced by said PEO processing is formed on the densified ceramic layer formed by the thermal spray process.