Plating turbine engine components

A process is provided for plating a coating onto a gas turbine engine component without detrimentally effecting air flow through cooling holes by injecting a maskant into the cooling passage to fill the cooling holes with the maskant, plating the external surface of the component with a coating, then removing the maskant from the component.

BACKGROUND OF THE INVENTION 
This invention relates to a process for plating gas turbine engine 
components, more particularly it relates to the masking of cooling holes 
in a gas turbine engine component during the plating process. 
The blades and vanes which are commonly used in the turbine section of 
modern gas turbine engines are typically made of nickel and cobalt based 
superalloys. The composition of the superalloys are generally tailored to 
provide a desirable combination of mechanical strength and resistance to 
environmental degradation (e.g. oxidation and hot corrosion). Coatings are 
often used to increase the level of oxidation and hot corrosion 
resistance, allowing the components made from such superalloys to be used 
for long periods of time before they need to be replaced or repaired. 
Such protective coatings can typically be applied by plating wherein an 
article is immersed in a plating medium. One problem faced by this coating 
technique is the deposition of the coating in unwanted areas. A variety of 
techniques have been developed to prevent coatings in undesired areas 
including the use of film forming polymeric resinous materials to protect 
the metal surface as is disclosed by U.S. Pat. No. 3,451,902. See also 
U.S. Pat. Nos. 2,999,771, 4,089,686 and 4,224,118. 
In gas turbine engines various components, in particular the high 
temperature turbine blades and vanes, are invariably air cooled to permit 
operation of the engine at a higher temperature. This air cooling requires 
the use of complex air cooling passages and cooling holes in the blades 
and vanes. In the application of protective coatings to such blades and 
vanes there is the tendency of the coatings to enter the cooling hole 
passages and have a detrimental effect on air flow. This problem has been 
observed in plating processes, e.g. platinum plating, wherein the platinum 
enters into and overlaps the hole opening thereby plugging the hole and 
having a serious consequence on air flow therefrom. Some holes are 
observed to be completely plugged, while the plugging of other holes 
affects air flow by 10% to higher than 50%. 
Various techniques which have been used in the art to deal with the hole 
plugging problem of plated blades have included: drilling the holes to a 
larger opening prior to coating to account for the subsequent plating; 
redrilling the holes after the plating has taken place; or sticking wires 
into the holes during the plating process. These methods are generally 
considered to be unsatisfactory, because they are time consuming and 
generally inefficient. 
SUMMARY OF THE INVENTION 
Briefly, a process is provided for plating a coating onto a gas turbine 
engine component which contains a plurality of cooling holes and a cooling 
passage comprising injecting a maskant into the cooling passage of the 
component to fill the cooling holes with the maskant, plating the external 
surface of the component with the coating, and removing the maskant from 
the component.

DETAILED DESCRIPTION OF THE DRAWINGS 
A process is provided for plating a coating onto a gas turbine engine 
component containing a plurality of cooling holes and a cooling passage 
interconnected therewith. Components containing such cooling passages and 
cooling holes include blades, vanes and shrouds. 
The first step of the process involves injecting a maskant into the cooling 
passage(s) of the component in order to fill the cooling holes with the 
maskant. As shown in FIGS. 1 and 3 typically, for blades 1 and vanes the 
cooling passages 2 are accessed and the injection is carried out through 
the root 3. In order for the maskant to effectively fill the cooling holes 
4 it may be necessary to preheat the component, and insert the hot molten 
maskant into the cooling passages under pressure effective to fill the 
cooling holes. As shown in FIGS. 4 and 5 the cooling holes 4 are filled so 
that the maskant 5 is flush with the surface of the component. The maskant 
is preferably an organic material which will facilitate its application 
and subsequent removal. The maskant is used to prevent coating of the 
metallic surface areas it is in contact with during plating and should not 
detrimentally react with the metal surface of the component or interfere 
with the plating bath. Plastics are preferred in that they can be 
injection molded into the component in a liquid state, then cured to 
harden the plastic for the subsequent plating process. Suitable maskants 
include polypropylene and a polyurethane oligomer mixture. Preferably the 
maskant will not contain halogens which could detrimentally react with the 
metal surface. When injecting the maskant care should be taken that the 
maskant is not present on surfaces intended to be coated. Any maskant that 
is present on the outside of the component is generally removed before 
plating. 
After the maskant is injected into the cooling holes and cured to harden, 
if required, then plating of the external surface of the component with 
the protective coating can be carried out. A preferred plating process is 
an electroplating process which is well known in the art. A preferred 
protective coating to be applied by the electroplating process includes 
noble metals such as platinum. The use of the maskant injected into the 
cooling holes during the plating process inhibits coating of the holes 
which detrimentally affects airflow. 
Following completion of the plating process, the maskant is removed. A 
preferred maskant and method for its removal includes a maskant which will 
volatize on the application of high temperatures for an effective period 
of time. Other maskants which can be used include those which are removed 
by solvents. Typically the maskant can be removed by heat treatment at 
about 1100.degree. F. to 1700.degree. F. for 15 to 30 minutes. Treatment 
at these temperatures will not detrimentally effect the superalloy surface 
of the gas turbine engine component. As shown in FIG. 2, without the 
maskant filling the cooling holes during plating, platinum will plate both 
the external surface 6 and the internal passages of the cooling hole 7 
which detrimentally affects air flow. With the maskant filling the cooling 
holes during plating the platinum will plate only the external surface 8 
as shown in FIG. 6. 
After removal of the maskant the component may then be processed as is 
common in the art, including a diffusion heat treatment to diffuse the 
protective coating, e.g. platinum, into the component's surface. Other 
operations may also be suitably carried out including the applications of 
additional coatings to the plated component. A preferred additional 
coating which is applied to a platinum coated substrate is a diffusion 
aluminide coating which can be applied by a vapor diffusion or pack 
diffusion (e.g. pack cementation) process followed by diffusion of the 
coating into the substrate at elevated temperatures (e.g. 1500.degree. to 
2000.degree. F.). 
EXAMPLE 1 
Polypropylene at a temperature of about 400.degree. F. and under a pressure 
of about 1000 psi is injected into the cooling passages 2 through the root 
3 of a CF6-80C2 first stage blade (see FIG. 3), filling the cooling holes 
4 of the blade. Excess polypropylene on the outside of the blade is 
cleaned off. The polypropylene sets to harden as it cools below about 
200.degree. F. The blade is then platinum plated in an electroplating bath 
containing a platinum diamminedinitrite solution at 180.degree. F. for 90 
minutes yielding a platinum thickness of 0.0002 to 0.0004 inches. 
Following plating the plastic is volitized by a burnout at 1100.degree. F. 
for 30 minutes, ultrasonic cleaning in 150.degree. F. water for 15 minutes 
and a water backflush for 5 minutes. 
The platinum plated parts were further coated by having a diffusion 
aluminide coating applied to the platinum plated surface by pack 
cementation and diffusion at 1800.degree. F. for 6 hours providing a 
platinum aluminide protective coating. The effect on airflow by hole 
plugging during platinum plating was measured with and without maskant 
injected into the cooling holes with the following observations. The 
average change in mass airflow is measured for each of the three chambers 
9, 10 and 11 in the turbine blade 1 depicted in FIG. 3, with Wa indicating 
the leading edge chamber 9, Wb indicating the central chamber 10 and Wc 
indicating the trailing edge chamber 11. 
The control (without maskant) showed an average change in mass airflow for 
each chamber over 5 different plating and coating runs as follows: 
Wa--49.3% 
Wb--27.8% 
Wc--22.8% 
The maskant injected blade showed an average change in mass airflow for 
each chamber over 5 different plating and coating runs as follows: 
Wa--12.1% 
Wb--8.6% 
Wc--7.7% 
The masked blades thus exhibited a dramatic improvement in airflow after 
platinum plating and coating compared to the control platinum plated and 
coated blades without use of maskant. 
EXAMPLE 2 
The process of Example 1 is repeated using a UV curable urethane acrylic 
polymer as the maskant which after injection is UV cured until hard and 
heat cured at 250.degree. F. for 30 minutes. 
The plated blades also exhibited open cooling holes with minimal airflow 
change.