Process for depositing an anti-wear coating on titanium based substrates
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the deposition of anti-wear coatings on titanium or titanium alloy parts, and the coated parts thus obtained. 
     2. Summary of the Prior Art 
     It is very difficult to obtain very adherent deposits on titanium and titanium alloy parts because of their high passivitiy. 
     It has already been proposed to subject such parts to be coated to a preliminary treatment before proceeding with the deposition of the actual anti-wear coating, so as to improve the adherence of the latter. Examples of the preliminary treatments which have been proposed are: 
     Anodic attack in mixtures of glycol--hydrofluoric acid, acetic acid; 
     Predeposition of zinc from glycol--metal fluoride mixtures, or aqueous mixtures based on fluoroboric, hydrofluoric acids and metal salts; 
     Long duration pickling in concentrated hydrochloric or sulphuric-hydrochloric acids, followed by deposition of iron, nickel or cobalt in a very acidic bath. 
     In all of these methods it is necessary or recommended to combine the preliminary treatment with a thermal treatment between 400° C. and 800° C. in an atmosphere which does not contaminate titanium, in order to improve the firmness of the metal coating. However, this firmness is never excellent. In particular, the coating obtained does not withstand machining or finish-grinding. 
     It has also been proposed in FR-A-1 322 970 to carry out a preliminary chemical treatment in an oxidixing medium, which comprises subjecting the part to the action of a bath of chromic anhydride, alkaline phosphate and hydrofluoric acid for from 5 to 30 minutes at a temperature between 35° C. and 100° C. However this treatment has the double drawback of generating hydrides in the coating and effecting an undesirable penetration of hydrogen into the substrate during subsequent electrolytic operations. 
     It is therefore an object of the invention to be free of the above-mentioned drawbacks by doing away with preliminary chemical treatments of the substrate, thereby avoiding the production of hydrides and curbing the penetration of hydrogen into the substrate during the electrolytic process of depositing the anti-wear coating. 
     It is also an object of the invention to enable an anti-wear coating to be deposited on titanium parts while reducing fatigue failure compared to the known processes, thus permitting the use of coated titanium substrates for parts subjected to cyclic fatigue, where such parts obtained by the known processes would not be acceptable. 
     A further object of the invention is to achieve a deposition sequence in which the technique of nickel deposition by magnetron cathodic spraying (cathode sputtering) so as to obtain a particularly adherent sub-layer on the substrate is associated with an electrolytic deposition permitting the deposition of a final anti-wear coating. 
     Yet another object of the invention is to define parameters for the deposition of nickel by cathodic spraying, compatible with subsequent electrolytic depositions. 
     SUMMARY OF THE INVENTION 
     According to the invention a process for depositing an anti-wear coating on a titanium-based substrate comprises the steps of: 
     a) roughening the substrate by sanding; 
     b) deposition of a keying nickel sub-layer on the substrate by cathodic spraying; 
     c) intermediate cleaning of the part obtained from step (b) 
     d) electrolytic activation of the cleaned part by immersion of the part in a cyanide bath; 
     e) electrolytic deposition of a layer of nickel on the activated part obtained from step (d); and, 
     f) deposition of a final, anti-wear layer of a material selected from the group consisting of Ag, Cr, Ni, Co, and mixtures of any two or more thereof, with or without ceramic particles such as SiC, Cr 2  C 3 , Al 2  O 3 , Cr 2  O 3 . 
     According to a preferred embodiment of the invention, step (b) comprises two successive sub-steps, (b 1 ) and (b 2 ), carried out in an inert gas atmosphere, the two sub-steps being: 
     b 1 ) ionic pickling of the substrate in a vacuum enclosure at a pressure between 1×10 -1  and 50 Pa; and 
     b 2 ) nickelling the substrate by cathodic spraying in an inert atmosphere, obtained by the introduction of argon into the vacuum enclosure, at a pressure between 2×10 -1  and 5 Pa, preferably between 0.4 and 0.8 Pa. 
     Preferably the cathodic spraying of sub-step (b 2 ) is carried out with a magnetron cathode. 
     The invention also provides a coated part comprising a titanium based substrate; a first coating layer of nickel deposited on said substrate by magnetron cathode spraying to a thickness ranging from 3 to 7 microns; a second coating layer of nickel deposited electrolytically on said first layer by prenickelling in an acid bath followed by nickelling in a sulphamate bath, said second coating layer having a thickness between 18 and 20 microns; and a final, anti-wear coating layer deposited on said second layer and comprising a material selected from the group consisting of Ag, Cr, Ni, Co, and mixtures of any two or more thereof, with or without ceramic particles such as SiC, Cr 2  C 3 , Al 2  O 3 , Cr 2  O 3 , said final coating layer having a thickness in excess of 80 microns. 
     Other characteristics of the process in accordance with the invention will become apparent from the following description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are graphs showing the results of rotary bending fatigue tests carried out on annular test pieces of TA6V treated in various different ways in accordance with the state of the art defined by FR-A-1 322 970 or in accordance with the invention, as indicated by the legends associated with the graphs, said graphs plotting the permissible rotary bending stresses against the number of cycles performed. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description the steps of the process in accordance with the invention will be explained, by way of example, with reference to depositions carried out on test pieces of TA6V titanium alloy in the cast state. The test pieces used were as follows: 
     bars of 30 mm diameter and 80 mm height; 
     plates measuring 100×20×2 mm; 
     bars having drilled holes of 30 mm diameter and 12 mm depth. 
     The first step is a roughening of the substrate, e.g. by sanding in the dry state with 50 micron corundum or by wet sanding with quartz of 40 microns, an operation shown by tests to be desirable to obtain a satisfactory adherence of the subsequently deposited nickel. 
     The part is then placed in a vacuum enclosure at a high secondary vacuum, i.e. at a pressure between 3×10 -4  and 3×10 -1  Pa, and the substrate is subjected to ionic pickling which cleans the substrate by removal of matter. To do this, the part is placed in an inert gas atmosphere, for example argon injected into the enclosure at a pressure between 1×10 -1  and 50 Pa, while a negative voltage is applied to the substrate so as to attract the ions to the substrate during the luminescent discharge carried out in the enclosure. The operation may be carried out within a power density range of from 0.05 to 0.4 W/cm 2 . Tests have shown that the preferred range is between 0.1 and 0.15 W/cm 2  for a perod of from 15 to 20 minutes. 
     After this ionic pickling operation the deposition of a keying layer of nickel on the substrate is effected. As mentioned earlier the method chosen for this deposition comprises cathodic spraying. This technique is a vacuum deposition process conducted in the cold state, in luminescent plasma, in a gas maintained at a reduced pressure of 0.1 to 10 Pa. The material to be deposited, nickel in this instance, is termed the target material and is introduced into the vacuum enclosure in the form of a plate of a few millimeters thickness, this being placed at the cathode position. The substrate is placed at the anode position. 
     At the residual pressure of the enclosure, the electric field created between the two electrodes gives rise to ionization of the residual gas which produces a luminescent cloud between the electrodes. The substrate then becomes covered with a layer of the same material as the target, due to the condensation of atoms originating from the target under the impact of positive ions contained in the luminescent gas and attracted by the target as a result of its negative polarization. 
     In preferred embodiments of the invention the deposition of the keying nickel is carried out by cathodic spraying with a magnetron cathode, so as to improve the quality of adherence of the nickel and increase the deposition rate to obtain an operating time compatible with the demands of industrial production. 
     With a magnetron cathode the electric field is combined with an intense magnetic field perpendicular to the electric field, that is to say parallel to the target. This superimposition of the two fields has the effect of winding the electron paths around the magnetic field lines, considerably increasing the chances of ionizing a gas molecule in the vicinity of the cathode. The ionization efficiency of the secondary electrons emitted by the cathode is increased as a result of the lengthening of their paths. This increase of ionic density in the proximity of the target brings about a substantial increase of the ionic bombardment of the latter, hence an increase of the quantity of atoms ejected for the same applied voltage. 
     Preferably the substrate to be coated, which is placed at the anode position, is polarized at a voltage between -20 and -500 V. The best results are obtained between -100 and -150 V. 
     The target is of pure nickel and is bombarded at a power density between 70 and 700 W/dm 2 , the power density for the bombardment of the target being selected depending upon the temperature admissible by the substrate to be coated. 
     Spraying is carried out in an inert atmosphere within a pressure range of from 0.2 to 5 Pa, the best results being obtained between 0.4 and 0.8 Pa. 
     To obtain a nickel deposit of from 5 to 7 microns, an operation time between 45 and 60 minutes is sufficient, which constitutes an appreciable advantage over previously used techniques which require several hours. 
     The part then undergoes an alkaline immersion degreasing operation for from 3 to 7 minutes (typically 5 minutes) in an aqueous bath containing from 30 to 45 g/l of Turco 4215 NCLT or from 40 to 60 g/l Ardrox PST 39 (registered trade marks), followed by rinsing in cold water with monitoring of the water film continuity. 
     Electrolytic activation of the part is then effected by dipping it for one minute in an aqueous bath containing from 60 to 80/l KCN and from 10 to 50 g/l K 2  CO 3  at a current density (c.d.) of from 1.5 to 3 A/dm 2 . 
     The part is then further rinsed in cold water, after which an electrolytic nickelling operation is performed. This is carried out in two successive stages: 
     (1) prenickelling in an acid bath (pH=1.1) under the following operational conditions: 
     temperature: 50°±5° C. 
     current density: 6±1 A/dm 2  for 3 minutes, then 4±1 A/dm 2  for 10 minutes 
     in a bath containing: 
     NiCl 2 .6H 2  O: from 280 to 350 g/l 
     Ni metal: from 69 to 86 g/l 
     H 3  BO 3  : from 28 to 35 g/l 
     The average deposited thickness is 15 microns, and the part is again rinsed in fresh water before the next stage. 
     (2) nickelling in a sulphamate bath under the following operational conditions: 
     temperature: 50°±5° C. 
     current density: 2 A/dm 2  for 5 minutes, then 4 A/dm 2  for 5 minutes 
     in a bath containing 
     Ni sulphamate: from 75 to 90 g/l 
     NiCl 2 , 6H 2  O: 18 g/l 
     chloride ion Cl: from 3.75 to 5.60 g/l 
     H 3  BO 3  : from 30 to 40 g/l 
     The thickness of nickel deposited ranges from 3 to 5 microns. 
     The part is then again rinsed in cold water before being given its anti-wear coating, for example of Cr, Ni-Co, Ni Co Sic or Ag-Ni. 
     As a first example, an electrolytic chromium coating may be obtained under the following working conditions: 
     temperature: 54°±1° C. 
     current density: 25 A/dm 2  for 10 minutes, then 20 A/dm 2  for 12 hours 
     in a bath containing: 
     CrO 3  : from 225 to 275 g/l 
     H 2  SO 4  : from 2 to 3 g/l 
     Cr +++  : from 2.5 to 8 g/l 
     with the ratio CrO 3  /H 2  SO 4  being between 90 and 120. 
     The average thickness obtained is between 120 and 150 microns. 
     As a second example, an anti-wear coating of Ni-Co containing 29% Co may be obtained using a bath in which the Ni/Co mass ratio is 20 and the total Ni+Co in solution is 87.5 g/l. 
     The nickel and the cobalt are introduced into the bath in the form of nickel sulphamate Ni (NH 2  SO 3 ) 2 , 4H 2  O and cobalt sulphamate Co (NH 2  SO 3 ) 2 , 4H 2  O, and are deposited under the following operational conditions: 
     temperature: 50°±2° C. 
     pH: 3.9±0.1 
     current density: 2 A/dm 2  for 10 minutes, then 4 A/dm 2  for 3 hours and 25 minutes. 
     The parts are placed on a rotary mounting and the bath stirred with compressed air. 
     The average coating thickness obtained is from 120 to 140 microns. 
     After receiving its anti-wear coating, the part is rinsed in cold water and then dried with compressed air, followed by degassing at 200°±5° C. for 3 hours. 
     In order to determine the fatigue resistance of titanium based parts coated with anti-wear deposits in accordance with the invention, rotary bending fatigue tests were conducted on annular test pieces. 
     For this purpose, test pieces coated in accordance with the invention were compared with test pieces coated according to the state of the art as taught by FR-A-1322970. 
     The tables included herein show the operations which were carried out. 
     Table 1 shows the treatment steps applied to 56 test pieces, some of which were left at various intermediate stages of the coating processes before subjecting them to the rotary bending fatigue tests. 
     Table 2 illustrates the precise operational conditions of the electrolysis carried out in the operations indicated in Table 1. 
     The curves of FIGS. 1 and 2 illustrate the results of the rotary bending fatigue tests, showing the variation of stresses as a function of the number of cycles according to the finished state of the parts, and depending on whether they were obtained by the invention or in accordance with the state of the art. 
     The curves show that the parts coated in accordance with the invention have a rotary bending fatigue failure rate much lower than those produced in accordance with the state of the art as illustrated by FR-A-1 322 970. 
     The permissible maximum stresses at the end of 10 8  cycles can be summarized as shown in the following table depending on whether the coatings (nickelling alone, Ni Co, or Cr) were obtained in accordance with the invention or according to the state of the art: 
     
         ______________________________________                                    
          Substrate                                                       
          TA 6 V                                                          
σ (MPa)                                                             
          (ref.)    Prenickelling                                         
                                Ni Co  Cr                                 
______________________________________                                    
STATE OF  500       200         170    250                                
THE ART                                                                   
INVENTION 500       380         380    440                                
______________________________________                                    
 
    
     Table 3 shows the results of vibratory fatigue tests carried out on a number of samples, depending upon the nature of the treatment used for each sample, the number of cycles, and the maximum stresses applied. 
     
                       TABLE 1                                                     
______________________________________                                    
TREATMENT STEPS APPLIED IN THE CASE OF                                    
ROTARY BENDING FATIGUE TEST PIECES                                        
TEST PIECE REFERENCES                                                     
                  TREATMENT STEPS                                         
______________________________________                                    
42-33-26-38-35-29-40                                                      
                  Polished ground condition +                             
                  Nickel PVD (1)                                          
64-65-66-67-68-69-70                                                      
                  Dry sanding +                                           
                  Nickel PVD (1)                                          
50-51-52-53-54-55-56                                                      
                  Dry sanding + Ni PVD +                                  
                  prenickelling pH = 1.1 +                                
                  sulphamate nickelling (1)                               
15-37-32-36-34-30-33                                                      
                  Dry sanding + Ni PVD +                                  
                  prenickelling pH = 1.1 +                                
                  sulphamate nickelling +                                 
                  nickel-cobalt (1)                                       
25-31-41-36-37-38-39                                                      
                  as above + chromium (1)                                 
9-10-11-12-13-14-16                                                       
                  Dry sanding + activation +                              
                  prenickelling pH = 1.1 +                                
                  sulphamate nickelling (2)                               
57-58-59-60-61-62-63                                                      
                  Same preparation steps +                                
                  chromium (2)                                            
17-19-20-21-22-23-24                                                      
                  As above + nickel-cobalt (2)                            
______________________________________                                    
 (1) Steps in accordance with the invention (nickel PVD + electrolytic    
 depositions)                                                             
 (2) Steps in accordance with the state of the art (chemical preparation +
 galvanization)                                                           
 
    
     
                                           TABLE 2                                 
__________________________________________________________________________
OPERATIONAL ELECTROLYSIS CONDITIONS (*) APPLIED                           
TO THE ROTARY BENDING FATIGUE TEST PIECES                                 
              Steps according to                                          
                               Steps in accordance                        
              the state of the art                                        
                               with the invention                         
              c.d.                                                        
                  I1  I2  duration                                        
                               c.d.                                       
                                   I1  I2  duration                       
BATHS         A/dm2                                                       
                  mA  mA  minutes                                         
                               A/dm2                                      
                                   mA  mA  minutes                        
__________________________________________________________________________
PRENICKELLING 8   540 816  5   7   476 714  3                             
pH = 1.1      4   272 408 10   4.5 306 459 10                             
NICKEL-SULPHAMATE                                                         
              2   135 204  5   2   136 204  5                             
              4   272 408  5   4   272 408  5                             
CHROMIUM      40  2720                                                    
                      4080                                                
                          10   25  1700                                   
                                       2550                               
                                           10                             
              35  2400                                                    
                      3500                                                
                           8h  20  1360                                   
                                       2040                               
                                            12h                           
NICKEL-COBALT 2   135 204 10   2   135 204 10                             
(29% cobalt)  4   272 408 3h25 4   272 408 3h25                           
__________________________________________________________________________
 (*) Rotary mounting                                                      
 I1 = 2 test pieces; I2 = 3 test pieces                                   
 
    
     
                                           TABLE 3                                 
__________________________________________________________________________
RESULTS OF VIBRATORY FATIGUE TESTS                                        
                      Maximum                                             
                            Drop in                                       
                                 Maximum                                  
                                       Drop in                            
                      stress                                              
                            Fatigue                                       
                                 stress                                   
                                       Fatigue                            
NATURE OF TREATMENT   10.sup.5 cycles                                     
                            limit                                         
                                 10.sup.8 cycles                          
                                       limit                              
__________________________________________________________________________
(1)                                                                       
  TA6V reference state                                                    
                      600 MPa                                             
                            /    520 MPa                                  
                                       /                                  
(2)                                                                       
  Polished ground state + Nickel PVD (1)                                  
                      570 MPa                                             
                             5%  420 MPa                                  
                                       19%                                
(3)                                                                       
  Dry sanding state + nickel PVD (1)                                      
                      550 MPa                                             
                             9%  420 MPa                                  
                                       19%                                
(4)                                                                       
  Dry sanding + nickel PVD + pre-                                         
                      550 MPa                                             
                             9%  400 MPa                                  
                                       23%                                
  nickelling + nickel sulphamate (1)                                      
(5)                                                                       
  Same as treatment (4) + nickel-                                         
                      480 MPa                                             
                            20%  380 MPa                                  
                                       27%                                
  cobalt 0.1 mm + 3 h 200° C. (1)                                  
(6)                                                                       
  Same as treatment (4) + chromium                                        
                      520 MPa                                             
                            13%  440 MPa                                  
                                       15%                                
  0.1 mm + 3h 200° C. (1)                                          
(7)                                                                       
  Dry sanding + activation + pre-                                         
                      400 MPa                                             
                            33%  200 MPa                                  
                                       61%                                
  nickelling + nickel sulphamate (2)                                      
(8)                                                                       
  Same as treatment (7) + nickel-                                         
                      300 MPa                                             
                            50%  170 MPa                                  
                                       67%                                
  cobalt 0.1 mm + 3 h 200° C. (2)                                  
(9)                                                                       
  Same as treatment (7) + chromium                                        
                      280 MPa                                             
                            53%  250 MPa                                  
                                       52%                                
  0.1 mm + 3h 200° C. (2)                                          
__________________________________________________________________________
 (1) Steps in accordance with the invention (nickel PVD + electrolytic    
 depositions)                                                             
 (2) Steps in accordance with the state of the art (chemical preparation +
 electrolytic depositions)                                                
 
    
     If a comparative analysis is made of the results of these tests on parts having a similar level of coating, depending on whether the coating is obtained in accordance with the invention or according to the state of the art, the following points may be seen: 
     The drop in fatigue limit after 10 8  cycles of parts having undergone only the nickelling (thus without the final coating) is 61% if the part is obtained according to the state of the art, but only 23% if the part is obtained by nickel PVD, then electrolysis as proposed by the invention. 
     In the coated state, for 0.1 mm thick chromium coatings, the drop in fatigue limit is 52% for parts produced according to the state of the art and only 15% for the parts produced in accordance with the invention. 
     For 0.1 mm thick nickel-cobalt coatings, the difference is even greater as the drop in fatigue limit is 67% for parts coated according to the state of the art and 27% for the parts coated in accordance with the invention. 
     The results discussed above show that the invention enables the lowering of the fatigue limit to be appreciably limited for parts coated with protective deposits relative to the uncoated substrate, as regards both vibration fatigue and rotary bending fatigue. 
     Thus, by providing an industrially exploitable process (by virtue of its comparatively limited duration relative to the state of the art) for making reliable and durable anti-wear coatings on titanium alloy substrates, the invention enables coated titanium parts to be produced for use in restrictive environments where such parts could not previously be used. 
     It is therefore possible to use titanium substrates, which are very much lighter than the materials normally used, for parts subjected to lasting fatigue stresses, due both to rotary bending and to vibrations.