Coating turbine blades with boron

Erosion caused by airborne particles and pollutants wears the airfoils of gas turbines used in industril and aerospace applications, resulting in gradual loss of cross-section and airfoil thickness. To compensate for cross-section loss and subsequent deterioration of the aerodynamics of the compressor blades, more fuel is consumed to maintain the required energy output.

A thin layer of hard wear-resistant coating can maintain efficiency for a longer period of time and result in considerable fuel cost savings over the life of the engine. Treating metallic components with boron is one way to improve wear resistance; however, some boron treatments result in surfaces that are brittle and crack easily, according to a US patent given to Willian Hurst and Glenn Cauthren of X-Treme Aerospace, Inc. Below are details from the patent.

Boron has been previously used as an element of turbine blades, particularly at the tip of the blade. Application of boron to the entire turbine blade will improve the hardness and durability over the entire body of the turbine blade, increasing the useful life of the component as a whole. 

The boron may be applied through a variety of technologies, and to a variety of turbine blades, such as titanium blades, carbon steel blades, and stainless steel blades. 

Any type of iron-based steel alloy can be treated with boron to increase hardness and/or durability, such as carbon steel, tool steel, and stainless steel. Treatment of turbine blades with boron results in the formation of a boron alloy layer 20 at the surface of the turbine blade. The boron alloy layer 20 does not extend through the whole cross section of the treated component, leaving a portion of the base metal 22 unaltered. With respect to stainless steel (or any steel) turbine blades, this alloy layer is substantially composed of FeB and Fe.sub.2B.

An alloy of iron and boron makes up about 90% to about 100% of the boron alloy layer after treatment of a stainless steel turbine blade. Small amounts of other elements present in the turbine blade comprise the remaining 0% to 10% of the alloy layer. Treatment of a stainless steel turbine blade results in an boron alloy layer that is predominantly iron boride in part because of the composition of the stainless steel used in the blades and part because boron will not form an alloy with some of the other elements used in stainless steel. 

While titanium turbine blades may also be comprised of various titanium alloys, such as 6/4 titanium, the boron alloy layer created by treating a titanium turbine blade with boron is substantially composed of titanium boride. The vanadium and aluminum do not form alloys with the boron, so the resulting boron alloy layer has small amounts of other elements, including elemental boron, but is almost all titanium boride. 

While treating metal components, such as turbine blades, with boron increases the hardness of the components, layers of pure boron applied on the surface of such components can be brittle. However, by diffusing boron into the base metal of the component, rather than layering it on top, hardness and durability can be increased without an accompanying brittleness. A desired thickness of the boron alloy layer on a titanium or stainless steel turbine blade is in the range of about 0.01 to about 0.001 inches thick. A thickness within this range will provide increased hardness for the turbine blades, while minimizing the brittle qualities of the boron.