A method of coating a plurality of non-planar substrates with synthetic diamond material using a microwave plasma chemical vapour deposition synthesis technique, the method comprising: forming a composite substrate assembly comprising: a support substrate comprising an upper surface; a plurality of electrically conductive refractory guards disposed over the upper surface of the support substrate and extending to a height h g above the upper surface of the support substrate; and a plurality of non-planar substrates disposed over the upper surface of the support substrate and extending to a height h s above the upper surface of the support substrate, wherein the height h s is less than the height h g, wherein a difference in height h g - h s lies in a range 0.2 mm to 10 mm; placing the composite substrate assembly within a plasma chamber of a microwave plasma CVD reactor; feeding process gases into the plasma chamber including a carbon containing gas and a hydrogen containing gas; feeding microwaves in the plasma chamber to form a microwave plasma at a location over the composite substrate assembly; and growing synthetic diamond material on the plurality of non-planar substrates, wherein each of the non-planar substrates comprises a convex dome with a peripheral skirt extending from an edge of the convex dome and the synthetic diamond material is disposed over the entire surface of the convex dome and extends down at least a portion of the peripheral skirt, and wherein the plurality of non-planar substrates are removed from the synthetic diamond material after growth to yield a plurality of free-standing non-planar polycrystalline CVD synthetic diamond components. forming a composite substrate assembly comprising: a support substrate comprising an upper surface; a plurality of electrically conductive refractory guards disposed over the upper surface of the support substrate and extending to a height h g above the upper surface of the support substrate; and a plurality of non-planar substrates disposed over the upper surface of the support substrate and extending to a height h s above the upper surface of the support substrate, wherein the height h s is less than the height h g, wherein a difference in height h g - h s lies in a range 0.2 mm to 10 mm; a support substrate comprising an upper surface; a plurality of electrically conductive refractory guards disposed over the upper surface of the support substrate and extending to a height h g above the upper surface of the support substrate; and a plurality of non-planar substrates disposed over the upper surface of the support substrate and extending to a height h s above the upper surface of the support substrate, wherein the height h s is less than the height h g, wherein a difference in height h g - h s lies in a range 0.2 mm to 10 mm; placing the composite substrate assembly within a plasma chamber of a microwave plasma CVD reactor; feeding process gases into the plasma chamber including a carbon containing gas and a hydrogen containing gas; feeding microwaves in the plasma chamber to form a microwave plasma at a location over the composite substrate assembly; and growing synthetic diamond material on the plurality of non-planar substrates, wherein each of the non-planar substrates comprises a convex dome with a peripheral skirt extending from an edge of the convex dome and the synthetic diamond material is disposed over the entire surface of the convex dome and extends down at least a portion of the peripheral skirt, and wherein the plurality of non-planar substrates are removed from the synthetic diamond material after growth to yield a plurality of free-standing non-planar polycrystalline CVD synthetic diamond components.