One major goal of plasma spraying and plasma treatment of materials includes generation of stable plasmas having the capability to control, within a relatively wide range, the heat and momentum transfer to feedstock thus providing desirable parameters (temperature, velocity, etc.) of feedstock. This in turn provides for the formation of a deposition with required properties in a required area of a part to be sprayed. Additional goals may include control of substrate temperature as well as other conditions of a deposit formation.
In plasma spraying, it is often the case that parts identified for coating may have geometries and areas with relatively limited access where conventional plasma torches may not be efficiently utilized because of their dimensions. Non-limiting examples include internal surfaces of tubes having relatively small diameters of about several centimeters and relatively narrow spaces inside turbine transition pieces or between airfoils used in turbine power generation. For example, the space between airfoils in a first stage nozzle may be about 40 mm or even smaller. Moreover, certain areas targeted for spraying may be relatively difficult to view or not even possible to view, which leads to significant technical challenges to provide an efficiently spray pattern and a relatively high quality coating.
There have been attempts to provide plasma torches and systems for spraying areas having limited accessibility. Examples include: (1) Model 2700 manufactured by Praxair-Tafa and Thermach; (2) Model SG-2100 manufactured by Praxair-Tafa; (2) F210 and F300 manufactured by Oerlicon Metco; and (3) Model 100HE manufactured by Progressive Surface.
FIG. 1 is an illustration of the Model 2700 plasma torch manufactured by Thermach. As shown therein, the plasma torch includes a connecting and distributing module 302, straight extension 304 and plasma torch module 306 which contains the cathode and anode. Extension 304, which is therefore prior to the plasma torch, may enclose for example, power leads, incoming and returning water lines and plasma gas lines. A powder feeding line 312 locates outside of the extension 304. Outside lines supplying cooling air and other means needed to cool spraying surface and to remove or deflect dust are not shown. As may be appreciated, the accessibility of Model 2700 will depend upon the diameter and length of extension 304. Presently, common lengths of the extension 304 is understood to be about 300 mm to 600 mm with diameters of about 21 mm to 26 mm. These torches are also understood to operate at electrical power of below 30-32 kW, having thermal efficiency η of about 0.5 or less generating argon based plasmas having enthalpies of 10-12 kJ/g or below. Relatively low power, low η and relatively low enthalpy plasmas having values of <12 kJ/g result in relatively low feedstock spray rates of about 20-25 g/min and, relatively often, low deposition efficiency and quality of sprayed coatings.
Attention is next directed to U.S. Pat. No. 4,661,682 entitled “Plasma Spray Gun For Internal Coatings” which is described for insertion in pipes and bores of work pieces and for coating the internal surfaces of said work pieces. Reference is also made to U.S. Pat. No. 5,837,959 entitled “Single Cathode Plasma Gun With Powder Feed Along Central Axis Of Exit Barrel” which describes a plasma gun in which powder is introduced into the gun is entrained in a plasma stream for deposit on a workpiece spaced from the gun. With reference to FIG. 6A of U.S. Pat. No. 5,837,959, the anode arc root attachment is identified at 104 as the spot on the anode wall where the arc 100 terminates. Downstream from such location, there are generally observed significant heat losses to the point where the plasma exits the torch. There is therefore a trend to minimize the length of such a plasma passage (the distance from the anode arc root attachment to the point where the plasma exits the torch) to a value of 30-40 mm or less.
Attention is next directed to U.S. Pat. No. 4,853,515 entitled “Plasma Gun Extension For Coating Slots.” The Abstract indicates that the plasma gun for spraying in a recessed region comprises a cathode member and a tubular anode arranged with the cathode member to generate a plasma stream. An elongated tubular extension including a tubular wall with an axial plasma duct therein extends forward of the anode. The tubular extension is described to have a length of 12.5 cm from the cathode tip to an end wall at the end of the plasma duct.
Attention is next directed to U.S. Pat. Nos. 9,150,949 and 9,376,740. As disclosed therein, systems, apparatus and methods have become available for plasma spraying and plasma treatment of material based upon high specific energy plasma gases that may be used to generate a selected plasma.
Accordingly, a need remains for a plasma torch and method that would allow for delivery of plasma and spray material at an location extended from the anode arc root attachment of ≥150 mm, via use herein of a plasma extension module (PEM) optionally including a nozzle module (NM) wherein the enthalpy of the plasma that exits the PEM at the extended location (HEXIT) has an value of ≥15 kJ/g.