Ultrahard multilayer coating comprising nanocrystalline diamond and nanocrystalline cubic boron nitride
A multilayer coating (MLC) is composed of two chemically different layered nanocrystalline materials, nanodiamond (nanoD) and nano-cubic boron nitride (nono-cBN). The structure of the MLC and fabrication sequence of layered structure are disclosed. The base layer is preferably nanoD and is the first deposited layer serving as an accommodation layer on a pretreated substrate. It can be designed with a larger thickness whereas subsequent alternate nano-cBN and nanoD layers are typically prepared with a thickness of 2 to 100 nm. The thickness of these layers can be engineered for a specific use. The deposition of the nanoD layer, by either cold or thermal plasma CVD, is preceded by diamond nucleation on a pretreated and/or precoated substrate, which has the capacity to accommodate the MLC and provides excellent adhesion. Nano-cBN layers are directly grown on nanodiamond crystallites using ion-assisted physical vapor deposition (PVD) and ion-assisted plasma enhanced chemical vapor deposition (PECVD), again followed by nanodiamond deposition using CVD methods in cycles until the intended number of layers of the MLC is obtained.
1. A multilayer coating formed on a substrate, the multilayer coating comprising: a base layer, and a plurality of nano-cBN and nanodiamond layers deposited on the said base layer in alternate sequence, wherein the cBN nanocrytallites nucleate directly on nanoD crystallites in said plurality of nano-cBN and nanodiamond layers.
2. A multilayer coating as claimed in claim 1 wherein said base layer is a layer of nanodiamond and the first layer in said alternate sequence is a layer of nano-cBN.
3. A multilayer coating as claimed in claim 1 wherein said base layer is a layer of single crystalline diamond or polycrystalline diamond and the first layer in said alternate sequence is a layer of nanodiamond.
4. A multilayer coating as claimed in claim 1 further comprising a buffer layer between the substrate and the base layer.
5. A multilayer coating as claimed in claim 1 wherein the plurality of alternating layers each have a thicknesses ranging from 1 to 100 nm.
6. A multilayer coating as claimed in claim 1 wherein the plurality of alternating layers terminate with a top nano-cBN layer.
7. A multilayer coating as claimed in claim 1 wherein the plurality of alternating layers terminate with a top nano-diamond layer.
8. A multilayer coating as claimed in claim 1 wherein the substrate is selected from a group consisting of semiconductors, insulators, metals, and alloys.
9. A multilayer coating as claimed in claim 1 wherein the substrate is pre-treated.
10. A multilayer coating as claimed in claim 9 wherein the substrate is pre-treated by the processes of enhanced seeding in hard powder suspension, prescratching, chemical etching and/or bias enhanced nucleation before depositing said base layer.
11. A multilayer coating as claimed in claim 1 wherein the nanocrystalline diamond films are deposited by cold and thermal plasma CVD.
12. A multilayer coating as claimed in claim 1 wherein the first layer of nanodiamond is polished prior to subsequent cBN deposition.
13. A multilayer coating as claimed in claim 1 wherein the nano-cBN layers are deposited by ion-assisted physical vapor deposition (PVD) and/or plasma-enhanced (or ion assisted) chemical vapor deposition (PECVD).
14. A multilayer coating as claimed in claim 1 wherein the nano-cBN layers are deposited by cold or thermal plasma CVD.
15. A multilayer coating as claimed in claim 14 wherein a negative bias is applied to the substrate during said deposition processes.
16. A multilayer coating as claimed in claim 1 wherein the coating stress is lower than that of individual nano-cBN components with identical thickness.