Coupling apparatuses and methods of forming the same

A method of forming a coupling apparatus including a first component and a second component adapted to contact the first component in shear engagement, the method comprising:

• providing a first metal substrate of the first component, wherein the first metal substrate comprises a titanium-based material;
• providing a second metal substrate of the second component;
• forming a protective coating on at least one of the first metal substrate or the second metal substrate, wherein the protective coating consists of:
• a first contact layer having a thickness of less than or equal to about 5 microns and comprising material that is inert to titanium, wherein the first contact layer has a contact surface that is adapted to directly contact an opposing surface in shear engagement;
• and
• optionally, a diffusion barrier layer disposed between the first contact layer and the corresponding metal substrate.

A turbine engine shaft comprising:

• a compressor rotor comprising a first metal substrate, wherein the first metal substrate comprises a titanium-based material;
• a turbine rotor comprising a second metal substrate, wherein the turbine rotor is adapted to contact the compressor rotor in shear engagement and wherein the second metal substrate comprises metal chosen from nickel, iron, titanium, alloys thereof, or mixtures thereof;
• a protective coating disposed directly on at least one of the first metal substrate or the second metal substrate, wherein the protective coating consists of:
• a physical vapor-deposited contact layer having a thickness of less than or equal to about 5 microns and comprising material that is inert to titanium, wherein the physical vapor-deposited contact layer has a contact surface that is adapted to directly contact an opposing surface in shear engagement and wherein the physical vapor-deposited contact layer comprises material chosen from gold, platinum, cobalt, or alloys thereof;
• cobalt oxide, boron oxide, or boron nitride;
• or mixtures thereof;
• and
• optionally, a diffusion barrier layer disposed between the physical vapor-deposited contact layer and the corresponding metal substrate;
• wherein the compressor rotor and the turbine rotor are adapted to contact in shear engagement through a curvic coupling configuration, with the contact surface of the physical vapor-deposited contact layer in shear engagement with the opposing surface in the curvic coupling configuration.