METHOD OF FORMING A MOLYBDENUM-CONTAINING PHOSPHATE SURFACE-COATING MATERIAL

Abstract: A method of using an electrochemical process involving oil-based lubricants to form friction-reducing and wear-reducing molybdenum-­ containing phosphate films on metal surfaces such as cast iron surfaces. A cast iron surface acting as an anode means is first coated with a thin film of molybdenum and then bombarded with high energy inert gas ions to form a layer of molybdenum/iron alloy on the surface before the electrochemical process takes place. A dialkyl hydrogen phosphate is added to the oil-based lubricant to improve its electrical conductance and to make it into an ionic conductor. This method enables not only the formation of uniform molybdenum/iron phosphate films on cast iron surfaces but also the in situ repair of such films after damage has occurred.

Description: Field of the invention This invention generally relates to a method of forming friction-reducing and wear-reducing phosphate films on metal surfaces immersed in an oil-based lubricant and, more particularly, is concerned with a method of using an electrical potential in oil-based lubricants containing a dialkyl hydrogen phosphate additive to form phosphate films on metal surfaces as specified in the preamble of claim 1, for example as disclosed in US-A-4,714,529. Background of the invention In the automotive industry, engine friction and wear reduction by adding additives to an oil-based lubricant is one of the most appealing approaches for improving vehicle fuel economy. The potential benefits realizable from this technique can be readily applied to all vehicles at low cost. Additives have been added to oil-based lubricants to reduce friction and wear by creating chemical reactions between the additives and the metal surfaces lubricated by the lubricant. However, these chemical reactions are very slow and the reaction films thus formed are generally not uniform. It has also been proposed that metal surfaces may be coated with friction-reducing or wear-reducing films before they are assembled into a vehicle. However, the durability of these films is less than desirable and it is extremely difficult to repair damaged films after a vehicle has been assembled and operated. The formation of wear-reducing films on metal surfaces immersed in aqueous solutions or molten salts by an electrochemical technique has been performed by others. Great difficulties were encountered when the same electrochemical technique was used in a process to form friction-reducing and wear-reducing films on metal surfaces immersed in oil-based lubricants. Since oil-based lubricants have extremely high electrical resistance, electrical current cannot pass through the lubricants. In US-A-4,714,529 issued December 22, 1987, an electrolytic additive of dialkyl hydrogen phosphate was added to an oil-based lubricant and iron phosphate films were formed on cast iron surfaces. However, the durability of these films formed was found to be inadequate in motor vehicle engine applications. It is therefore an object of the present invention to provide a method of coating metal surfaces immersed in oil-based lubricants with a friction-reducing and wear-reducing film that has improved durability. A method of forming a friction-reducing and wear-reducing film on a metal surface according to the present invention is characterised by the features specified in the characterising portion of claim 1. The present invention provides a method of coating metal surfaces immersed in oil-based lubricants with a durable friction-reducing and wear-reducing film electrochemically by pretreating the metal surface with a molybdenum film and bombarding the surface with inert gas ions. Summary of a preferred embodiment of the invention A cast iron surface is first cleaned by an oxygen plasma for five minutes to eliminate all the contaminants thereon. A film of molybdenum having a thickness of approximately 30 nanometres is then coated on the cast iron surface by a vapour deposition method. The molybdenum and the iron substrate are then reacted together by bombarding the iron surface with 200 keV krypton ions to form an iron-molybdenum (Fe-Mo) alloy on the surface. This Fe-Mo alloy-coated cast iron surface is then subjected to an electrochemical process immersed in an oil-based lubricant to convert the iron particles into iron phosphate. It has been discovered that, in order to coat metal surfaces immersed in oil-based lubricants with a friction-reducing and wear-reducing film and to make in situ repair of such films possible, an electrochemical process rather than a chemical process must be utilized. An added benefit of the electrochemical process is that films of more uniform and greater thickness than those produced by a chemical process may be formed. In order to apply the electrochemical process to form friction-reducing and wear-reducing films on metal surfaces immersed in oil-based lubricants, the lubricant must first become a component of the electrochemical system, i.e. an ionic conductor. Since oil-based lubricants have extremely high electrical resistance, electrolytic additives must be blended into the oil-based lubricants at a desirable concentration to reduce the electrical resistance thereof and to improve the electrical conductance thereof. Stable open-circuit potential readings are thus obtained and electrical current is passed between the metal surfaces. It has been discovered that suitable electrolytic additives for oil-based lubricants are dialkyl hydrogen phosphates, specifically, dilauryl hydrogen phosphate (DHP) and mixed-alkyl acid orthophosphates. For instance, friction-reducing and wear-reducing iron phosphate films were detected on surfaces of cast iron electrodes after immersion in an oil-based lubricant containing 2.5 wt% dilauryl hydrogen phosphate and after being subjected to electrolysis. This suggests that the formation of a uniform friction-reducing and wear-reducing film and in situ repair of damaged films on sliding surfaces are possible. Brief description of the drawings Other objects, features and advantages of the present invention will become apparent upon consideration of the specification and the appended drawings, in which: Figure 1 is a schematic diagram showing a flow chart of an electrochemical surface coating process according to the present invention. Figure 2 is a schematic diagram of an electrochemical charging cell used in the present invention. Figure 3 is a graph showing the atomic concentration on a cast iron specimen after step 1, the molybdenum coating step, obtained by Auger Electron Spectroscopy. Figure 4 is a graph showing the atomic concentration on a cast iron specimen after step 2, the iron alloying step, obtained by Auger Electron Spectroscopy. Figure 5 is a graph showing the atomic concentration on a cast iron specimen after step 3, the electrochemical step, obtained by Auger Electron Spectroscopy. Figure 6 is a graph showing the friction-reducing effect of a molybdenum/iron phosphate film formed on the cast iron surface by the electrochemical process of the present invention. Detailed description of a preferred embodiment The reduction of engine friction through lubricant modification is one of the most appealing approaches for improving vehicle fuel economy because the technique can be readily applied to all vehicles at low cost. Previous work has shown that additive interaction with lubricated sliding surfaces plays an important role in the reduction and control of friction and wear. It is known that additives used to reduce friction and wear are usually long straight-chain organic molecules having a polar group at one end. The magnitude of the polarity is an important factor in its performance in reducing friction. It has been discovered that, in order to coat metal surfaces immersed in oil-based lubricants with a friction-reducing and wear-reducing film of sufficient thickness in a relatively short period of time, an electrochemical process rather than a chemical process must be utilized. Additional benefits of using the electrochemical process are that films of more uniform and greater thickness may be formed than those produced by a chemical process and that in situ repair of these friction-reducing and wear-reducing films may be possible. It is commonly believed that oil-based lubricants are insulators rather than ionic conductors and therefore such lubricants are poor components for electrochemical systems. It has been discovered that, in order to apply an electrochemical technique to the formation of friction-reducing and wear-reducing films on metal surfaces immersed in an oil-based lubricant, the lubricant must first become a component of the electrochemical system, i.e. an ionic conductor. As one of the novel features of the present invention, dialkyl hydrogen phosphates, specifically dilauryl hydrogen phosphate and mixed-dialkyl acid orthophosphate, are used to change the ionic character of the oil-based lubricant. For instance, friction-reducing and wear-reducing iron phosphate films of sufficient thickness are formed on surfaces of cast iron electrodes after immersion in an oil-based lubricant containing 2.5 wt% dilauryl hydrogen phosphate and after being subjected to electrolysis. Another novel feature of the present invention is that, in order to significantly improve the durability of the iron phosphate film formed, a process of pre-coating the cast iron surface with a thin film of molybdenum is used. In this pre-coating process, a cast iron surface is first cleaned by oxygen plasma for five minutes to eliminate all impurities and contaminants thereon. A thin film, approximately 30 nanometres thick, of molybdenum is then coated on the cast iron surface by a vapour deposition method. This type of vapour deposition method is well described in the chemical literature, e.g. VLSI Technology, edited by S.M. Se, published 1983, p.350 to p.353. It is believed that a minimum thickness of molybdenum coating is required that is approximately 10 to 15 nanometres in thickness. In the next step of the pre-coating process, the molybdenum and the iron particles are reacted together in the surface layer of the cast iron specimen by bombarding the surface with 200 keV krypton (Kr) ions at a dosage rate of 5x10¹⁶ Kr ions/cm². After the bombardment, an iron/molybdenum alloy is formed on the surface of the cast iron specimen. There is reason to believe that other inert gas ions of similar or larger particle size such as argon, xenon, and radon should work equally well in place of krypton. For the bombardment method, equipment made by Varian/Extrion, Model CF 3000, is used. A maximum energy of 200 keV is applied to the krypton ions. The equipment is normally used for the processing of silicon wafers, but it is easily adapted for the surface alloying step of the present invention. The pre-coating process is shown in Figure 1 as step 1 and step 2 schematically in a flow chart. Step 3 illustrates the electrochemical step of the present invention that follows the coating and the alloying steps. The lubricant used in the present invention is a blend of a base stock, CITGO 90105, mineral oil and an additive of dilauryl hydrogen phosphate (DHP). The additive dilauryl hydrogen phosphate used in the present invention is of reagent grade obtained from the Mobil Corporation, U.S.A.. Referring now to Figure 2, an electrochemical cell composed of two iron electrodes 10 embedded in a ceramic sleeve 12 is shown. The purpose of the ceramic sleeve 12 is to help maintain lubricant 22 between the electrodes 10 and to maintain uniform current distribution. Since the electrical conductivity of lubricant systems in general is extremely low, i.e. smaller than 10⁻¹⁰ (ohms-cm)⁻¹, the two electrodes 10 have to be very closely spaced to each other in order to lower the ohmic resistance. A micrometer 14 is used to control the distance between electrodes 10. It has been found that a suitable distance to be used is 0.015 cm. The micrometer 14 is connected to one of the electrodes through insulating block 26 and set screws 28. The cast iron electrodes 10 in Figure 2 are of 25.4 mm (1″) diameter. One of the two electrodes is used as the anode, and the other is used as the cathode. The choice of electrodes is arbitrary. The roughness of the electrodes is less than 1 micrometre. The thickness of the electrodes is not critical. Before the electrolysis takes place, the electrodes 10 are rinsed with acetone to remove any grease on the surfaces. Electrode leads 20 (Fig.2) are made of stainless steel to collect current passing through the electrodes 10 and the lubricant 22 contained in ceramic sleeve 12. The size of the leads is not critical because of the extremely small amount of current passing through the leads. Two ceramic blocks 24 are used to insulate the two cast iron electrodes 10 from the container (not shown) of the electrochemical cell which is made of stainless steel. The test lubricant is prepared by mixing 97.5 grams of CITGO 90105 mineral oil with 2.5 grams of DHP. Examples : All tests were conducted at 23°C. The electrode surfaces used were analyzed using scanning electron spectroscopy, back-scattered electron spectroscopy, Auger Electron Spectroscopy (AES), electron spectroscopy for chemical analysis and X-ray diffraction. The lubricants used were also analyzed by infra-red spectroscopy, and nuclear magnetic resonance. Two cast iron electrodes 10 (Fig. 2) mounted in an electrochemical cell were galvanostatically polarized at a current of 0.05 micro-amp. After passing 108 micro-Coulomb of charge, the electrodes were removed for analysis. In order to identify the films deposited thereon, both the anode and the cathode were analyzed by the Auger Electron Spectroscopy (AES). A current density of 0.05 micro-amp/cm² has been used in the formation of iron phosphate film on the surface of 25.4 mm (1 inch) diameter iron electrodes in the novel process of the present invention. Experimental data indicated that any current density within the range of 0.001 micro-amp/cm² to 1000 micro-amp/cm² should work equally well. The current density to be used was determined by the length of the charge time desired, i.e. the smaller the current density, the longer the charge time required. Thus, for example, for a current density in the range of 0.01 to 100 micro-amp/cm², the length of the charge time ranges between 1 to 200 minutes in order to obtain a suitable thickness of the iron phosphate film. It has been found, for example, that, at a current density of 0.05 micro-amp/cm², a total charge time of approximately 36 minutes was required to form an iron phosphate film that was 150 nanometres thick. Other dialkyl hydrogen phosphates have also been used as the electrolytic additive in the oil-based lubricants to form friction-reducing and wear-reducing films. For instance, a mixed-dialkyl acid orthophosphate commercially available from DuPont under the tradename of Ortholeum 162 was successfully used to form iron phosphate films in the oil-based lubricants. It has also been found that a broad percentage range of the dialkyl hydrogen phosphate can be used as the electrolytic additive, i.e. between 0.1 to 99 weight percent. After the step 1 process, the cast iron surface coated with 30 nanometres thick molybdenum was examined by Auger Electron Spectroscopy. A depth profile obtained by the Auger Electron Spectroscopy is shown in Figure 3 which confirms the thickness of the molybdenum coat on the surface. In the depth profile shown in Figure 3, oxygen was also observed. It is believed that it was oxygen that originated from the oxygen plasma cleaning process conducted prior to molybdenum deposition. After the step 2 iron alloying process, the cast iron surface was examined again under the Auger Electron Spectroscopy. A quite different depth profile was obtained as shown in Figure 4. It is seen that iron has moved towards the surface whilst molybdenum has penetrated deeper into the substrate to a depth of more than 60 nanometres. Even though iron has not migrated all the way to the surface, a partial alloying of the iron and molybdenum was accomplished by the krypton ion bombardment. After step 3, the electrochemical step, a depth profile by the Auger Electron Spectroscopy was again obtained on the cast iron surface. This is shown in Figure 5. The appearance of phosphorus and oxygen in the depth profile is evidence of the formation of iron phosphate on the specimen surface. When compared to Figure 4, Figure 5 shows lower iron concentration near the surface of the specimen. Since an accelerated corrosion reaction occurs during the electrochemical step of the process of the invention, it is believed that iron atoms located near the surface of the specimen could have been etched away. The observation of carbon in Figure 5 implies a possible hydrocarbon dispersion in the inorganic iron phosphate structure. In a commercial SAE 30 engine oil, the molybdenum/iron phosphate coating provides excellent results in reducing friction. This is shown in Figure 6. The friction coefficient for molybdenum/iron phosphate coated specimens remains within a range from 0.070 to 0.074 for at least 300 hours. Even after 400 hours of sliding motion, the friction coefficient for molybdenum/iron phosphate coated specimens is still as low as 0.087. In contrast, the friction coefficient for an iron phosphate-coated specimen started to increase after 50 hours due to the wear of the surface coating and the friction coefficient becomes identical to that for uncoated specimens after 80 hours. These results demonstrate that even under severe test conditions, molybdenum/iron phosphate films are at least six times more durable than the iron phosphate film. These results suggest that molybdenum/iron phosphate films are promising coating materials for metal surfaces in motor vehicle applications, such as in piston rings and other moving components, for reducing engine friction. Furthermore, in situ repair of damaged films on sliding metal surfaces is also possible by using the process of the invention periodically during the operation of the system. For instance, this technique may be applied to the in situ repair of molybdenum/iron phosphate films formed between sliding components in an internal combustion engine, i.e. the piston ring and the cylinder surface. It should be noted that even though the exemplified tests were conducted on molybdenum-coated specimens bombarded with krypton ions at a maximum power level of 200 keV, there is reason to believe that ions of other inert gases having similar or larger particle sizes such as argon, xenon and radon should function equally well at power levels between 50 keV to 5000 kev. For ions such as argon, which have a smaller particle size, a higher power level than 200 keV is necessary. For ions having larger particle sizes such as xenon and radon, a lower power level than 200 keV may be utilized.

Claims: 1. A method of forming a friction-reducing and wear-reducing film on a metal surface by an electrochemical process comprising the steps of making the metal surface the anode (10) of an electrochemical cell, spaced apart from a cathode (10), both of which are immersed in an oil-based lubricant (22), blending a dialkyl hydrogen phosphate into said oil-based lubricant sufficient to allow electrical current to pass in said lubricant, and passing an electrical current of sufficient current density between said anode (10) and cathode (10) so that said friction-reducing and wear-reducing film is formed on said metal surface, characterised in that the method includes first pre-coating said metal surface with molybdenum and then bombarding the coated surface with inert gas ions prior to submitting said metal surface to said electrochemical process so that a molybdenum-containing phosphate film is formed on said metal surface. 2. A method of forming a friction-reducing and wear-reducing film on a metal surface according to claim 1, characterised in that the metal surface is that of a cast iron anode means (10) spaced apart from said cathode (10), and said friction-reducing and wear-reducing film is formed substantially of molybdenum/iron phosphate on the surface of said cast iron anode means (10). 3. A method of forming a friction-reducing and wear-reducing film on a metal surface according to claim 1 or 2, characterised in that the dialkyl hydrogen phosphate is either dilauryl hydrogen phosphate or a mixed-dialkyl acid orthophosphate. 4. A method of forming a friction-reducing and wear-reducing film on a metal surface according to claim 1 or 2, characterised in that between 0.1 to 99 weight percent of the dialkyl hydrogen phosphate is blended into said oil-based lubricant, and an electrical current having a current density between 0.001 to 1000 micro-amp/cm² is passed between said anode (10) and said cathode (10) for a length of time sufficient to form said friction-reducing and wear-reducing film. 5. A method of forming a friction-reducing and wear-reducing film on a metal surface according to claim 1 or 2, characterised in that said metal surface is coated with molybdenum to a thickness of no less than 10 nanometres and is bombarded with krypton ions prior to immersion in said oil-based lubricant, between 1 to 5 weight percent of either dilauryl hydrogen phosphate or a mixed-dialkyl acid orthophosphate is blended into said oil-based lubricant, and an electrical current having a current density between 0.01 to 100 micro-amp/cm² is passed between said anode (10) and said cathode (10) for a period of time between 1 to 200 minutes in order to obtain said friction-reducing and wear-reducing film on said metal surface.

Inventors: TUNG, SIMON CHIN-YU, WANG, SU-CHEE SIMON
Applicants: GENERAL MOTORS CORPORATION
Application year: 1989
Main classification: C25D11
IPC codes: C10M105, C25D11