This paper deals with study of the global thermoelastic instability of brake disk made of either the isotropic homogeneous metal matrix composite (MMC) or the isotropic inhomogeneous functionally graded material composite (FGM). The main idea of the FGM composite is a smooth variation of material properties due to continuous change in microstructure. The estimates in diffusion equation are characterized both by non‐stationary heat flux and non‐stationary heat convection terms enhanced by the characteristic frictional heat sources. Global instability is characterized by the Schaefer–Papkovich condition. Application of functionally graded A356R‐based composite to brake disk structure prevents loss of global stability in contrast with homogeneous A356R composite and stainless steel ASTM321 brake disk which guaranties safety and durability of the braking system.