rus
Issue #8/2014
B.Krit, N.Morozova, I.Ryzhikov, V.Savva, O.Somov, I.Suminov, A.Epelfeld
Nanostructured Polymer Ceramic Coatings for Sealing Ends
Nanostructured Polymer Ceramic Coatings for Sealing Ends
The hybrid technology of the surface synthesis
of nanostructured composite ceramic-polymer
coatings allows to obtain high-quality
end seals of high-precision, durable, low-torque
and fast-running mechanisms.
of nanostructured composite ceramic-polymer
coatings allows to obtain high-quality
end seals of high-precision, durable, low-torque
and fast-running mechanisms.
Теги: micro-arc discharge oxidation surface engineering vapor deposition polymerization газофазная депозитная полимеризация инженерия поверхности микродуговое оксидирование
Surface engineering is a relatively new trend in science and technology including both traditional and innovative ways of modifying the surface of products. Surface engineering covers many areas of modern material sciences and is based on the processes designed to generate surface layers with the required properties directly in the substrate material and coat by connecting through various methods of firmly associated other layers usually of composite material with properties differing from those of the substrate. An interest in surface modification is also due to the fact that in most cases the surface characteristics in combination with the properties of the substrate material determine the product properties in general. In addition, surface modification is more attractive as compared with methods for changing the bulk properties of materials and products from the economic point of view [2].
Formation of end seals,
technologies and problems
Development of new environmentally friendly technologies for the creation of multi-functional modification of surface layers and coatings protecting and reinforcing metal products are the relevant issues of modern science and technology. An example of this approach is the engineering of mating surfaces in the creation of sealing ends. A mechanical seal of the contact type is used in machines and mechanisms for separating the cavities of high and low pressures. Usually it is in the form of a pair of friction surfaces of mechanical seals of two parts, one of which is mounted on the rotor, and the other one on the stator (housing) of the device. A mechanical seal is also a kind of thrust bearings with a friction pair for the operation of which lubrication needs to be supplied into the space between the rubbing surfaces. The difference between mechanical seals and conventional thrust bearings in the nature of operation is that the rubbing pair of mechanical seals should operate under pressure drops, and in most cases a sealing medium is used as a lubricant. Mechanical seals are the most effective and durable type of sealing media of the rotating shafts of pumps and other machines, and have significant benefits in terms of the quality of sealing and durability as compared for example with gland seals [3].
In many cases, increasing the durability of friction pairs is achieved by a variety of methods of formation on the mating surfaces of rather thick (more than 30 µm) protective oxide layers. Recently, for this variety of engineering surfaces increasingly used are the methods of plasma exposure in electrolytes, a relatively new type of the electrochemical surface treatment of mainly metal materials [4]. Modifying with an electrolyte plasma micro-discharges allows synthesising on valve group metals surfaces (Al, Mg, Ti, Zr, Nb, Ta and some others, during the anodic oxidation of which the oxide films with unipolar conduction in the metal-oxide-electrolyte system are formed on the surface) the nanostructured oxide composite layers, which in many respects are superior to the coatings created in other ways [5]. A combination of functional properties (high hardness, wear and corrosion resistance, electrical insulation properties etc.) makes it possible to use the modified products in many industries. However, during the development and application of this process it should be taken into account that the residual and through-porosities in the modified layer, which are necessary for the physicochemical mechanisms leading to plasma formation and synthesis of the desired oxide phases, are an integral feature of the process. In a case when forming a porous structure on the surface (for example, to retain grease or ensure thermal protection) is not an objective, the porosity should be considered as a disadvantage which significantly reduces the material and product characteristics not allowing to fully achieve the desired modification results [6]. To regulate the residual porosity (including its complete elimination) and give the surface the consumer properties, various kinds of additional processing take place. Among these it can be mentioned impregnation or filling of open pores with various substances (organic and inorganic liquids, polymers, metal melts); fusion; mechanical processing etc., which can seal the open pores and significantly extend the application of oxidised parts.
Let us consider the features of using of one of plasma electrolytic methods, micro-arc discharge oxidation (MDO) in combination with vapor deposition polymerization (VDP) [6] to fill the residual open porosity. Such an integration of the modification methods provides the basis for developing a new hybrid technology of surface synthesis of the composite nanostructured ceramic-polymer coatings proven for the manufacture of mechanical seals of high-precision, durable and rapidly rotating low-torque mechanisms. The use of the MDO layer of the VDP process for filling the pores opens up new surface engineering opportunities because fundamentally new nanostructured ceramic polymer-composite coatings are created on the surface of materials as a result of such combined treatment.
Equipment and methods of creation and evaluation of the oxide layer parameters
As a material for the production of prototypes chosen was a deformable thermally hardenable aluminium alloy D16 (3.8–4.9% Cu; 1.2–1.8% Mg; 0.3–0.9% Mn) commonly used in the industry. A synthesis of the nanostructured oxide-ceramic surface MDO layers was carried out on the MDO-100 unit of the MATI [4,5]. Oxidation was carried out in an alkaline silicate electrolyte (9 g/l Na2SiO3∙9H2O + 2 g/l NaOH) in the anode-cathode mode with a ratio of anode and cathode currents Ic/Iа = 1 and the total current density of 10 A/dm2 for 1–300 min.
The pores of the MDO layer with a polymer material (poly-para-xylylene) were filled on a vapor deposition polymerization unit developed at the Institute for Theoretical and Applied Electromagnetics RAS. Poly-para-xylylene is an aromatic polymer created as a thin-film highly adhesive coating with a thickness from 10 nm to 100 µm on the surface of substrates of various nature. The installation for the VDP process consists of three series-connected heated areas and a polymerisation reactor with a volume of 40 litres. In the area of sublimation is pre-loaded the paracyclophane dimer and the installation is vacuumized to a pressure in the polymerisation reactor of about 1 Pa. As a result of heating of sublimation area up to 150°C sublimation of paracyclophane dimer occurs, and its vapours get into the pyrolysis zone (600–700°C), where each paracyclophane dimer molecule splits into two para-xylylene molecules. Passing through the intermediate cooling zone (200°C) para-xylylene molecules get into a polymerisation reactor where they condense on the relatively cold (25°C) surfaces and are polymerised with a creation of a poly-para-xylylene film. The film is created simultaneously on the entire surface of the substrate irrespective of its profile, and creates a layer of uniform thickness. The polymer coating has the same good quality on the sharp edges, in holes and crevices, open pores, cracks and other hard-to-reach places.
The thickness of the modified layer was measured using an eddy-current thickness gauge BT-201 which is designed for fast, accurate and non-destructive testing the thickness of non-metallic coatings applied to a metal non-magnetic base (aluminium, copper, titanium etc.). To measure microhardness the Leitz micro-durometer with the Vickers diamond indenter were used. The load on the indenter was 50 g. To determine the porosity of the oxide composite layers was applied the methodology developed at the Department of the Materials-Processing Technology with High-Energy Streams of MATI and described in [5].
Tribological sample tests were performed on the upgraded friction machine CMT-1 equipped with a test unit for testing the friction pairs. The unit operates as follows (fig.1): a movable ring sample (1) with a thickness of 10 mm mounted in a holder and rotated at a pre-determined speed; non-movable ring sample (2) with a thickness of 12 mm mounted in the opposite holder having a force measuring sensor; recording samples is provided with a clamp (3). In the tests were recorded the slip speed, friction factor and load on the sample. The sliding speed was measured by the photosensor, the force of friction and friction load by means of strain gauges. To process signals from the recording sensors was developed the system for data collection, which is based on the special software and hardware of the National Instrument. The friction factor was calculated by the formula:
,
where N – normal load, N; F – friction force on the mean diameter, N.
The calculated ratio for the wear rate:
,
where l – linear wear, mm; Sf – sliding distance, mm.
The samples were tested in terms of friction and wear at a specific load of 0.5 MPa without lubrication, thus simulating the extreme conditions for using mechanical seals.
Growth kinetics, microhardness and porosity of the oxide layer
Fig.2 shows the kinetic dependence of the thickness of the oxide-ceramic layer during MDO. It should be noted almost a linear increase in the thickness of the tested time interval (300 min), with an average rate of 0.6µm/min. However, the growth rate as a differential characteristic varies within a fairly wide range from 0.4 to 1.1 µm/min. This is apparently due to the peculiarities of the oxidation mechanism and, above all, due a different effect of the temperature and time characteristics of the micro-arc discharge on the composition, structure and properties of the coating at different stages of MDO.
Fig.3 shows the dependence of microhardness Hµ and through-porosity Ps on the MDO coating thickness. Changing the microhardness of MDO coatings with an increase in their thickness can be explained as follows. When the coating thickness is about 50 µm, the conditions are established to start creating the working part of the modified layer [4, 5]. Thickening the modified layer up to about 100 µm is accompanied by an increase in the microhardness by more than 3 times (with stabilisation at about 900 kg/mm2) due to the synthesis in coating of the high-temperature phase α-Al2O3 (aluminium oxide) having a high hardness. With further increase in the thickness of the MDO layer begins the heat insulation of its working part with an outer sublayer, which is accompanied by a more complete high-temperature polymorphic transformation and increasing of microhardness to about 1,700 kg/mm2 at a coating thickness of about 150 µm.
On the curve showing the dependence of the through-porosity on the thickness of such MDO coatings (fig.3), we can distinguish two minima:
at the level 3–4% for MDO coatings with a thickness of about 100 µm;
at the level 5–6% for coatings with a thickness of 150–180 µm.
In this context it is important to point to the necessary through-porosity at any stage of MDO.
Filling the pores of MDO coatings of poly-para-xylylene by VDP can significantly reduce the amount of through-porosity [6].
Tribological properties
From the tribological point of view, the oxide-ceramic MDO coatings with a thickness of 150–180 µm can are of interest [5]. They are characterised by the highest values of microhardness and relatively low through-porosity, which can be reduced to almost zero (within the measurement error) by filling with poly-para-xylylene. Therefore, further research was carried out on the samples with coatings of the initial through-porosity of 5–6% at the thickness of the MDO layer of about 170 µm.
The pairs of samples with different nature of surface modification were subjected to the tribological tests:
oxide ceramics (MDO coating) – oxide ceramics;
oxide ceramics – ceramic-polymer (MDO coating with filling the residual porosity with poly-para-xylylene through the VDP method);
ceramic-polymer – ceramic-polymer.
For each of these combinations five pairs of samples were tested.
Fig.4 shows the test results of the friction pair oxide ceramics – oxide ceramics. The average wear rate of the MDO coating of the rotating (thin) samples was
I1 = 17.0∙10-8 mm/mm, and fixed – I2 = 13.9∙10-8 mm/mm. The average friction factor was f = 0.60. There should be noted significant oscillation values of the friction factor during the test.
Fig.5 shows a typical diagram corresponding to the friction test of the pair oxide ceramics – ceramic-polymer. In the course of this experiment the moving samples were coated with oxide ceramics, the fixed ones with ceramic-polymer. The coating on the movable samples was rubbed nearly to the substrate, and the wear amount was significantly higher compared with the fixed samples. The average wear rate of the coating on the rotating samples was I1 = 10.5∙10-8 mm/mm, and on the fixed ones
I2 = 0,88∙10-8 mm/mm. The mean friction factor after the final run-in (approximately 40 minutes) was f = 0.26.
Fig.6 demonstrates the results of tests of the friction pair ceramiс-polymer – ceramiс-polymer. The average wear rate of coating on the rotating samples was
I1 = 6,18∙10-8 mm/mm, and the fixed ones I2 = 0,83∙10-8 mm/mm. The average value of the friction factor was f = 0.08. This makes it possible to attribute the friction pair ceramic-polymer – ceramic-polymer to the anti-friction category because f ≤ 0.15 [5].
For testing the obtained experimental results a mechanical seal was made to seal the outlet end of the rotor shaft of a multi-centrifugal pump for pumping water, and its full-scale tests was conducted (fig.7). The mating parts were treated by the MDO method followed by filling the open residual pores of the oxide-layer with poly-para-xylylene (friction pair ceramic-polymer – ceramic-polymer). The rotation rate of the drive motor was 3,000 rpm, water pressure in the seal area is up to 16 kgc/cm2.
The test results showed that the torque at startup was 0.011 N∙m, the axial play at the conjugation area is less than 0.001 mm. During the performance tests for 200 hours the seal retained the full tightness (no leaks).
Prospects for the hybrid technology
In conclusion, it can be stated that the best results for the examined pairs of friction were obtained in the case of the contacting ceramic polymer surfaces. Interesting is a rather significant difference in the rate of wear of moving and stationary samples for all the examined combinations of body and counterbody. Perhaps this is due to the testing method and equipment used in the experiment.
With regard to the developed hybrid technology of the surface synthesis of the nanostructured composite ceramic-polymer coatings, the results indicate the prospects of its use for surface engineering during the creation of high-precision mechanical seals, durable, fast-moving and low-torque mechanisms.
The study was supported by RFBR (research project No.13-08-12038) and the Federal targeted programme for research and development in priority areas of development of the Russian scientific and technological complex for 2007-2013 (SC No. 14.513.11.0034) ■
Formation of end seals,
technologies and problems
Development of new environmentally friendly technologies for the creation of multi-functional modification of surface layers and coatings protecting and reinforcing metal products are the relevant issues of modern science and technology. An example of this approach is the engineering of mating surfaces in the creation of sealing ends. A mechanical seal of the contact type is used in machines and mechanisms for separating the cavities of high and low pressures. Usually it is in the form of a pair of friction surfaces of mechanical seals of two parts, one of which is mounted on the rotor, and the other one on the stator (housing) of the device. A mechanical seal is also a kind of thrust bearings with a friction pair for the operation of which lubrication needs to be supplied into the space between the rubbing surfaces. The difference between mechanical seals and conventional thrust bearings in the nature of operation is that the rubbing pair of mechanical seals should operate under pressure drops, and in most cases a sealing medium is used as a lubricant. Mechanical seals are the most effective and durable type of sealing media of the rotating shafts of pumps and other machines, and have significant benefits in terms of the quality of sealing and durability as compared for example with gland seals [3].
In many cases, increasing the durability of friction pairs is achieved by a variety of methods of formation on the mating surfaces of rather thick (more than 30 µm) protective oxide layers. Recently, for this variety of engineering surfaces increasingly used are the methods of plasma exposure in electrolytes, a relatively new type of the electrochemical surface treatment of mainly metal materials [4]. Modifying with an electrolyte plasma micro-discharges allows synthesising on valve group metals surfaces (Al, Mg, Ti, Zr, Nb, Ta and some others, during the anodic oxidation of which the oxide films with unipolar conduction in the metal-oxide-electrolyte system are formed on the surface) the nanostructured oxide composite layers, which in many respects are superior to the coatings created in other ways [5]. A combination of functional properties (high hardness, wear and corrosion resistance, electrical insulation properties etc.) makes it possible to use the modified products in many industries. However, during the development and application of this process it should be taken into account that the residual and through-porosities in the modified layer, which are necessary for the physicochemical mechanisms leading to plasma formation and synthesis of the desired oxide phases, are an integral feature of the process. In a case when forming a porous structure on the surface (for example, to retain grease or ensure thermal protection) is not an objective, the porosity should be considered as a disadvantage which significantly reduces the material and product characteristics not allowing to fully achieve the desired modification results [6]. To regulate the residual porosity (including its complete elimination) and give the surface the consumer properties, various kinds of additional processing take place. Among these it can be mentioned impregnation or filling of open pores with various substances (organic and inorganic liquids, polymers, metal melts); fusion; mechanical processing etc., which can seal the open pores and significantly extend the application of oxidised parts.
Let us consider the features of using of one of plasma electrolytic methods, micro-arc discharge oxidation (MDO) in combination with vapor deposition polymerization (VDP) [6] to fill the residual open porosity. Such an integration of the modification methods provides the basis for developing a new hybrid technology of surface synthesis of the composite nanostructured ceramic-polymer coatings proven for the manufacture of mechanical seals of high-precision, durable and rapidly rotating low-torque mechanisms. The use of the MDO layer of the VDP process for filling the pores opens up new surface engineering opportunities because fundamentally new nanostructured ceramic polymer-composite coatings are created on the surface of materials as a result of such combined treatment.
Equipment and methods of creation and evaluation of the oxide layer parameters
As a material for the production of prototypes chosen was a deformable thermally hardenable aluminium alloy D16 (3.8–4.9% Cu; 1.2–1.8% Mg; 0.3–0.9% Mn) commonly used in the industry. A synthesis of the nanostructured oxide-ceramic surface MDO layers was carried out on the MDO-100 unit of the MATI [4,5]. Oxidation was carried out in an alkaline silicate electrolyte (9 g/l Na2SiO3∙9H2O + 2 g/l NaOH) in the anode-cathode mode with a ratio of anode and cathode currents Ic/Iа = 1 and the total current density of 10 A/dm2 for 1–300 min.
The pores of the MDO layer with a polymer material (poly-para-xylylene) were filled on a vapor deposition polymerization unit developed at the Institute for Theoretical and Applied Electromagnetics RAS. Poly-para-xylylene is an aromatic polymer created as a thin-film highly adhesive coating with a thickness from 10 nm to 100 µm on the surface of substrates of various nature. The installation for the VDP process consists of three series-connected heated areas and a polymerisation reactor with a volume of 40 litres. In the area of sublimation is pre-loaded the paracyclophane dimer and the installation is vacuumized to a pressure in the polymerisation reactor of about 1 Pa. As a result of heating of sublimation area up to 150°C sublimation of paracyclophane dimer occurs, and its vapours get into the pyrolysis zone (600–700°C), where each paracyclophane dimer molecule splits into two para-xylylene molecules. Passing through the intermediate cooling zone (200°C) para-xylylene molecules get into a polymerisation reactor where they condense on the relatively cold (25°C) surfaces and are polymerised with a creation of a poly-para-xylylene film. The film is created simultaneously on the entire surface of the substrate irrespective of its profile, and creates a layer of uniform thickness. The polymer coating has the same good quality on the sharp edges, in holes and crevices, open pores, cracks and other hard-to-reach places.
The thickness of the modified layer was measured using an eddy-current thickness gauge BT-201 which is designed for fast, accurate and non-destructive testing the thickness of non-metallic coatings applied to a metal non-magnetic base (aluminium, copper, titanium etc.). To measure microhardness the Leitz micro-durometer with the Vickers diamond indenter were used. The load on the indenter was 50 g. To determine the porosity of the oxide composite layers was applied the methodology developed at the Department of the Materials-Processing Technology with High-Energy Streams of MATI and described in [5].
Tribological sample tests were performed on the upgraded friction machine CMT-1 equipped with a test unit for testing the friction pairs. The unit operates as follows (fig.1): a movable ring sample (1) with a thickness of 10 mm mounted in a holder and rotated at a pre-determined speed; non-movable ring sample (2) with a thickness of 12 mm mounted in the opposite holder having a force measuring sensor; recording samples is provided with a clamp (3). In the tests were recorded the slip speed, friction factor and load on the sample. The sliding speed was measured by the photosensor, the force of friction and friction load by means of strain gauges. To process signals from the recording sensors was developed the system for data collection, which is based on the special software and hardware of the National Instrument. The friction factor was calculated by the formula:
,
where N – normal load, N; F – friction force on the mean diameter, N.
The calculated ratio for the wear rate:
,
where l – linear wear, mm; Sf – sliding distance, mm.
The samples were tested in terms of friction and wear at a specific load of 0.5 MPa without lubrication, thus simulating the extreme conditions for using mechanical seals.
Growth kinetics, microhardness and porosity of the oxide layer
Fig.2 shows the kinetic dependence of the thickness of the oxide-ceramic layer during MDO. It should be noted almost a linear increase in the thickness of the tested time interval (300 min), with an average rate of 0.6µm/min. However, the growth rate as a differential characteristic varies within a fairly wide range from 0.4 to 1.1 µm/min. This is apparently due to the peculiarities of the oxidation mechanism and, above all, due a different effect of the temperature and time characteristics of the micro-arc discharge on the composition, structure and properties of the coating at different stages of MDO.
Fig.3 shows the dependence of microhardness Hµ and through-porosity Ps on the MDO coating thickness. Changing the microhardness of MDO coatings with an increase in their thickness can be explained as follows. When the coating thickness is about 50 µm, the conditions are established to start creating the working part of the modified layer [4, 5]. Thickening the modified layer up to about 100 µm is accompanied by an increase in the microhardness by more than 3 times (with stabilisation at about 900 kg/mm2) due to the synthesis in coating of the high-temperature phase α-Al2O3 (aluminium oxide) having a high hardness. With further increase in the thickness of the MDO layer begins the heat insulation of its working part with an outer sublayer, which is accompanied by a more complete high-temperature polymorphic transformation and increasing of microhardness to about 1,700 kg/mm2 at a coating thickness of about 150 µm.
On the curve showing the dependence of the through-porosity on the thickness of such MDO coatings (fig.3), we can distinguish two minima:
at the level 3–4% for MDO coatings with a thickness of about 100 µm;
at the level 5–6% for coatings with a thickness of 150–180 µm.
In this context it is important to point to the necessary through-porosity at any stage of MDO.
Filling the pores of MDO coatings of poly-para-xylylene by VDP can significantly reduce the amount of through-porosity [6].
Tribological properties
From the tribological point of view, the oxide-ceramic MDO coatings with a thickness of 150–180 µm can are of interest [5]. They are characterised by the highest values of microhardness and relatively low through-porosity, which can be reduced to almost zero (within the measurement error) by filling with poly-para-xylylene. Therefore, further research was carried out on the samples with coatings of the initial through-porosity of 5–6% at the thickness of the MDO layer of about 170 µm.
The pairs of samples with different nature of surface modification were subjected to the tribological tests:
oxide ceramics (MDO coating) – oxide ceramics;
oxide ceramics – ceramic-polymer (MDO coating with filling the residual porosity with poly-para-xylylene through the VDP method);
ceramic-polymer – ceramic-polymer.
For each of these combinations five pairs of samples were tested.
Fig.4 shows the test results of the friction pair oxide ceramics – oxide ceramics. The average wear rate of the MDO coating of the rotating (thin) samples was
I1 = 17.0∙10-8 mm/mm, and fixed – I2 = 13.9∙10-8 mm/mm. The average friction factor was f = 0.60. There should be noted significant oscillation values of the friction factor during the test.
Fig.5 shows a typical diagram corresponding to the friction test of the pair oxide ceramics – ceramic-polymer. In the course of this experiment the moving samples were coated with oxide ceramics, the fixed ones with ceramic-polymer. The coating on the movable samples was rubbed nearly to the substrate, and the wear amount was significantly higher compared with the fixed samples. The average wear rate of the coating on the rotating samples was I1 = 10.5∙10-8 mm/mm, and on the fixed ones
I2 = 0,88∙10-8 mm/mm. The mean friction factor after the final run-in (approximately 40 minutes) was f = 0.26.
Fig.6 demonstrates the results of tests of the friction pair ceramiс-polymer – ceramiс-polymer. The average wear rate of coating on the rotating samples was
I1 = 6,18∙10-8 mm/mm, and the fixed ones I2 = 0,83∙10-8 mm/mm. The average value of the friction factor was f = 0.08. This makes it possible to attribute the friction pair ceramic-polymer – ceramic-polymer to the anti-friction category because f ≤ 0.15 [5].
For testing the obtained experimental results a mechanical seal was made to seal the outlet end of the rotor shaft of a multi-centrifugal pump for pumping water, and its full-scale tests was conducted (fig.7). The mating parts were treated by the MDO method followed by filling the open residual pores of the oxide-layer with poly-para-xylylene (friction pair ceramic-polymer – ceramic-polymer). The rotation rate of the drive motor was 3,000 rpm, water pressure in the seal area is up to 16 kgc/cm2.
The test results showed that the torque at startup was 0.011 N∙m, the axial play at the conjugation area is less than 0.001 mm. During the performance tests for 200 hours the seal retained the full tightness (no leaks).
Prospects for the hybrid technology
In conclusion, it can be stated that the best results for the examined pairs of friction were obtained in the case of the contacting ceramic polymer surfaces. Interesting is a rather significant difference in the rate of wear of moving and stationary samples for all the examined combinations of body and counterbody. Perhaps this is due to the testing method and equipment used in the experiment.
With regard to the developed hybrid technology of the surface synthesis of the nanostructured composite ceramic-polymer coatings, the results indicate the prospects of its use for surface engineering during the creation of high-precision mechanical seals, durable, fast-moving and low-torque mechanisms.
The study was supported by RFBR (research project No.13-08-12038) and the Federal targeted programme for research and development in priority areas of development of the Russian scientific and technological complex for 2007-2013 (SC No. 14.513.11.0034) ■
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