Method for producing chromium-containing multilayer coating and a coated object
To produce a chromium-containing multilayer coating on an object, alternate layers of nickel phosphorus alloy and trivalent chromium are deposited on the object until a desired thickness of coating has been reached. The coated object is then subjected to one or more heat treatments to improve the mechanical and physical properties of the coating and to produce multiphase layers comprising layers containing crystalline Ni and crystalline Ni 3 P and layers containing crystalline Cr.
1. A method for producing a chromium-containing multilayer coating on an object, comprising the steps of:
depositing a layer of nickel-phosphorus alloy (NiP) on the object;
electroplating a layer of trivalent chromium on the object;
repeating said steps one or more times in order to produce a multilayer coating containing two or more alternate layers of nickel-phosphorus alloy and chromium; and
subjecting the coated object to one or more heat treatments to amend the mechanical and physical properties of the coating and to produce multiphase layers containing crystalline Ni and crystalline Ni 3 P and multiphase layers containing crystalline Cr.
2. A method according to claim 1 , wherein at least one of the chromium layers is deposited next to a NiP layer and during the one or more heat treatments at least one multiphase layer is produced that contains crystalline CrNi.
3. A method according to claim 1 , wherein the NiP layers are produced by electrodeposition or electroless deposition.
4. A method according to claim 1 , wherein the phosphorus content of the nickel phosphorus alloy is 1-15 w %.
5. A method according to claim 1 , wherein the phosphorus content of the nickel phosphorus alloy is 3-12 w-%.
6. A method according to claim 1 , wherein the phosphorus content of the nickel phosphorus alloy is 5-9 w-%.
7. A method according to claim 1 , wherein the temperature in the heat treatments is 200-1000° C.
8. A method according to claim 1 , wherein the temperature in the heat treatments is 400-750° C.
9. A method according to claim 1 , wherein the one or more heat treatments comprise at least two heat treatments carried out after the desired number of coating layers has been deposited on the object.
10. A method according to claim 1 , wherein the one or more heat treatments are carried out at a temperature between 500 and 700° C.
11. A method according to claim 1 , wherein the object is a metal object and hardening of the metal of the object is carried out at the same time as the coated object is heat-treated.
12. A method according to claim 11 , wherein the object is a steel object and the heat treatment is carried out at a temperature between 750 and 1000° C.
13. A method according to claim 1 , further comprising the step of depositing a top layer on the coated and heat-treated object by thin film deposition.
14. A method according to claim 1 , further comprising the step of depositing a top layer on the coated object by thin film deposition, before the step of subjecting the coated object to one or more heat treatments.
15. A coated object comprising a chromium-containing multilayer coating produced by the method according to claim 1 , the coating comprising multiphase layers containing crystalline Ni and crystalline Ni 3 P and multiphase layers containing crystalline Cr.
16. A coated object according to claim 15 , further comprising at least one multiphase layer containing crystalline CrNi.
17. A coated object according to claim 15 , further comprising at least one intermediate layer consisting of a metal or metal alloy other than NiP or Cr.
18. A coated object according to claim 17 , wherein the intermediate layer consists of nickel, copper, or molybdenum, or an alloy containing any of them.
19. A coated object according to any one of claim 15 , further comprising a top layer produced by thin film deposition.