Phosphating baths are mostly developed for microcrystalline single-phase coatings which can be precipitated with high reproducibility. The titanium phosphate pretreatment of steel promotes the formation of hopeite. This reaction leads to a deceleration of the covering process of the free surface and consequently to an increase in the amount of iron containing phosphophyllite. The different alloys and structures of the steel types is the reason for different rates of the pickling attack. Multiphase phosphate coatings containing phosphophyllite show improved tribological properties compared to zinc calcium phosphate coatings. This can be seen especially on the significantly decreased stick-slip inclination. The use of manganese phosphate coatings is to be preferred for many press-fit connections because they can be reproducibly precipitated, guarantee a higher torque transmission und successfully prevent tribo-oxidation.
1 Introduction, initial situation, objectives
Phosphate coatings are well-known and wellresearched conversion coatings. First patents were already filed as early as 1860. Phosphate coatings are still traditionally and successfully applied in temporary corrosion protection as carrier substrate for soaps, oils, fats, waxes and paints, in electrical engineering et al. . Relatively granular crystalline zinc phosphate has proved itself as thick carrier coating for drawing soap which facilitate cold-forming and has tool-protective effects. The more microcrystalline and thus thinner zinc calcium phosphate coatings are particularly preferred as undercoat for paints. In order to facilitate inflow processes, manganese phosphate as lubricant substrate has increasingly become accepted in recent years .
In 1980 conversion coatings with stable bonds in the form of microcrystalline phosphate coatings were applied for the first time, using the press-fit area of the shaft or the hub coated with a microcrystalline zinc calcium phosphate coating of about 3 to 5 μm (Phosphorsal P150, VEB Härtol-Werk Magdeburg). This new generation of press-fit connections was successfully applied, for example, in axle drives for the Athens Metro, the Berlin underground or in moving and slewing gear for excavators as well as in agricultural engineering [3-5]. The new generation of press-fit connections was exhibited at the Leipzig Trade Fair in 1987 and at the Hannover Trade Fair in 1996, where the application was recommended for the production of built camshafts as well.
With this kind of press-fit connections the following effects were achieved:
- increase in torque-transfer capability compared to conventional press-fit connections to about 200 % (coefficient of friction μ ≥ 0.2);
- prevention of tribo-corrosion (frictional corrosion) with local sliding in the press joint (more insensitive to slipping, microslipping);
- multiple non-destructive separation and renewed assembly;
- minimal tendency for adhesion;
- assembly as longitudinal press-fit connection without lubricants;
- special applicability for press-fit connections with thin-walled hubs, hollow shafts or both, because in this case only a relatively low joint pressure is possible and therefore a high coefficient of friction μ is necessary for a high transmission shearing stress τ = μ . p;
- high cost efficiency compared to form-fit shafthub connections;
- high reliability.
The power transmission in press-fit connections is determined primarily by the structural design. A suitable phosphatization of one or both partners increases the power transmission ability. The mechanisms are not yet completely clarified. Probably change of the phosphate coating characteristic occurs due to high pressures. The strength rises strongly and the intimate connection between steel and phosphate can lead to extreme local strengthening of the connection. This phenomena is well know e.g. from lubricants. The strength is larger with manganese phosphate than with zinc calcium phosphate coatings.
Fine-grained layers are proved as more favourable, because coarse-grained layers are homogeneous and ensure the formation of a closed phosphate pressing pattern between the partners of the pressfit connections.
Phosphorsal P150 is now no longer available and only test coatings with alternatives could be made. For this reason extensive scientific work has taken place at Chemnitz University of Technology to develop a new process technology and a new conversion coating that serves as a load-transfer element for forces and moments and prevents tribocorrosion. These coatings were examined in press-in and press-out tests as well as in static and dynamic changing-torsion tests on press-fit connections.
By means of intensive interdisciplinary research, TU Chemnitz succeeded in developing a new pressfit area coating with stable bonds. First the influence of an activation rinsing based on titanium phosphate suspension on the formation of zinc phosphate and zinc calcium phosphate coatings was examined. In this particular application the positive effect of phosphophyllite in zinc calcium phosphate coatings was revealed. Since the phosphophyllite content is difficult to adjust, other possibilities were looked for. Based on these experiments, the application of manganese phosphate coatings is preferred. Introduction in the industry was realised through co-operation with Paatz Viernau GmbH, which produce multispindle drill heads as their main product. Because of the higher efficiency parameters requested the form-fit feather key connections previously used were replaced by longitudinal pressfit connections with phosphated press-fit areas.
2 On the active principle of phosphating
Phosphate coatings are deposited preferentially from aqueous solutions through immersion or spray-on processes. In the following discussion a correct pre-treatment of the substrates is assumed which is essential for every coating technology. The formation of the phosphate coating finally depends on the tribasicity of the ortho-phosphoric acid and the formation of tertiary, insoluble phosphates. The chemical balance of an aqueous electrolytic solution containing free phosphoric acid, primary (and secondary) phosphates and special additives is disturbed through contact with a base metal which finally leads to the precipitation of insoluble phosphates and the formation of the phosphate coating. Depending on the existing conditions, coatings of varying composition, crystallinity and adherence to the substrate material are formed (Fig. 1).
Precipitation reactions, however, also lead to the formation of unwanted sludge at the bottom of the bath. The hydrogen arising at the iron is oxidized to water by means of oxidizing agents for example nitrite-, nitrate- or chlorate-based activators. Coating formation is accelerated and sludge deposition is reduced. The formation of the coating as a result of the etching reaction dies down when the covering of the free surface areas is completed. Therefore only limited film thicknesses can be obtained.
The following reactions take place in the zinc calcium phosphating of iron:
Consequently, the film shows different phases:
phosphophyllite (Zn2Fe(PO4)2·4H2O) and
or more dehydrated compounds depending on the film-drying conditions. Phosphating is normally optimized for the quick formation of microcrystalline dense coatings of low thickness. Single-phase coatings are preferentially formed, i.e. hopeite in zinc phosphate coatings, scholzite in zinc calcium phosphate, manganese-hureaulithe (Mn5H2(PO4)4) in manganese phosphate coatings. Commercial solution preparations for zinc calcium phosphate therefore contain the so-called embedded fine grainer. Microcrystalline films are obtained with zinc and manganese phosphating by means of suitable surface pre-treatment, especially through prerinsing with finely dispersed titanium phosphate or manganese phosphate respectively. Figure 2a and 2b impressively reveal the influence of pre-rinsing on the size of the crystallites in manganese phosphate coatings.
3 Influence of roughness, texture and pre-rinsing on the crystallite size
The aim of the experiment was to influence the chemical composition of the phosphate coatings Körin such a manner that they exhibit optimal properties for the application in press-fit connections and other load-transfer elements. This can be achieved if the coatings are microcrystalline on the one hand and dispose of sufficient bond stability on the other. Both criteria are determined by the choice of phosphating, but also by the choice of pre-treatment of the component parts (heat treatment, mechanical processing, cleaning and degreasing, etching, rinsing and activating).
Phosphate coatings were produced on cold-formed sheet metals (DIN 1623 T1) as well as on turned and machine-ground test specimens (C45N) by means of dipping application. After degreasing (P3 Upon, Henkel), hydrochloric acid pickling (1:1), water rinsing and to some extent activation rinsing for nucleation, commercial phosphating dispersions were used for phosphating. Dipping times were between 0.5 and 20 minutes. Layer morphology and crystallite sizes were documented by scanning electron microscopy (LEO 1455VP). X-ray diffraction (Siemens D5000) was applied to measure the phase ratio. The crystallite size was visually compared with the surface roughness determined by means of a profilometer (Hommel T4000, stylus radius 5 mm, stylus force 5 mN).
At first, discs were coated, the surfaces of which were ground on silicon carbide paper grade 80 and 600 or polished with alumina. Arithmetical mean deviations of the surface of 0.56 μm and 0.06 μm respectively are achieved through grinding and of 0.03 μm through polishing. The deposited phosphate coating levels roughnesses of the ground surface to a certain extent. However, it shows an arithmetical mean deviation of 0.2 μm even with deposition on the polished surface and is therefore characteristic for microcrystalline zinc calcium phosphate. Figure 3a to 3c shows this influence of the surface roughness on the morphology of the zinc calcium phosphate coatings.
Examination in the scanning electron microscope confirms that the arithmetical mean deviation is to be assigned directly to the size of the zinc calcium phosphate crystallites. Complete levelling of the chatter marks does not yet occur at a mass per unit area of 2.5 g/m2. It is particularly noticeable that the crystallites can be attributed to two different classes. This behaviour is even more pronounced the smoother the surface was before coating. Through pickling of the base material in order to develop the structure, it is verifiable that finer crystallites develop on perlite than on ferrite (Fig. 4a and 4b). This can be taken into account in the choice of the material and the design of heat treatment parameters for the component parts to be coated.
Figure 5 demonstrate the course of reaction in the starting phase of zinc calcium phosphating on a cold-rolled sheet metal (DIN 1623 T 1, material according to ISO 3574 Type CR1). After one to two seconds first crystal seeds are detectable (Fig. 5a and 5b) which will grow to 0.5 μm crystallite size after three seconds (Fig. 5c). After 5 seconds more than half of the surface is covered with granular phosphate crystallites which partly touch each other and consequently stop growing considerably bigger (Fig. 5d).
It is possible to obtain three-phase coatings with significantly bigger crystallites and therefore higher masses per unit area, film thicknesses and arithmetical mean deviation as is the case with conventional coatings. The phase ratio of the multi-phase conversion coatings is influenced by the duration of the pickling attack but also by the composition of the steel. Contrarily, the participation of the free surface with regard to the area and the time depends on the habit and the growth rate of the different crystalline phases such as hopeite, parascholzite and phosphophyllite because the crystallite forms and sizes influence the size of the free surface.
4 Influence of pre-rinsing on morphology and phase ratio
Other relations present when the surface is prerinsed with a titanium phosphate suspension. Such pre-treatment is regularly applied to produce more micro-crystalline zinc phosphate coatings through the creation of numerous seeds on the surface to be coated. In addition specialized companies offer pre-rinsing solutions which contain very small titanium phosphate and/or manganese phosphate crystallites. The number of these crystallites (influenced by treatment time, temperature and concentration) affects growth and size of the grains in the layer as well as the layer thickness.
Titanium phosphate pre-rinsing promotes the crystallisation of tabular zinc phosphate crystallites alongside the formation of granular zinc calcium phosphate. For geometrical reasons the covering of the free surface is delayed so that the pickling attack can take place over a longer period of time and additional zinc iron phosphate is formed (Fig. 6).
Without surface activation, i.e. seeding by titanium phosphate suspension, crystalline zinc phosphate coatings that develop through dipping in commercial dipping baths are quite granular crystalline. Not only zinc phosphate hydrates of different hydration states, which are confined to the hopeite at adequate drying temperature, are part of the phase ratio. The coatings include iron containing phosphophyllite as a further phase, which stands out in the SEM picture as radiating crystallites. These crystallites cause the strong roughness of the coating which is expressed by an arithmetical mean deviation of about 1 μm and a maximum surface roughness of 10 μm. The X-ray diffraction diagram is dominated by the (100) peak of the monoclinic phosphophyllite in contrast to the (002) peak of the orthorhombic hopeite and the (004) peak of the monoclinic zinc phosphate hydrate. Higher-induced phosphophyllite peaks point to a preferred grain alignment as illustrated in the secondary electron image of the surface (Fig. 7a and 7b). In addition, it is noticeable that a part of the steel surface is still uncovered despite a dipping time of 10 min. The sludge at the bottom of the bath consists of significant fractions of iron.
Fig. 7: Zinc phosphate with and without pre-rinsing; (a) X-ray diffraction diagram for 7b and c, zinc phosphate without pre-rinsing (b), zinc phosphate with pre-rinsing (c), (d) X-ray diffraction diagram for 7e und f (black – without pre-rinsing, blue – with pre-rinsing), zinc phosphate without pre-rinsing (e), zinc phosphate with prerinsing (f)
The activation rinsing with titanium phosphate leads to the growth of a great many very small tabular crystallites (Fig. 7c) which quickly completely cover the surface and therefore finish the pickling process during phosphating at an early stage. As a result, pure zinc phosphate coatings without iron content develop, which even occur in single-phase form after drying at 100 °C (only hopeite). The deposition of conversion coatings formed in this way is well reproducible. The grain refinement becomes apparent in the roughness parameters, e.g. the arithmetical mean deviation decreases from 1 μm to 0.5 μm. Roughness and porosity are still sufficiently high to absorb, for example, drawing soap or corrosion protection waxes. These zinc phosphate coatings prove to be unsuitable for press-fit area coating because strong stick-slip features occur already during press-in (chapter 5).
The stick slip effect develops on damped coupled surface exercising a fast motion sequence made of sticking, staying, separation and gliding. This leads depending on the tribological system to oscillations, which are radiated from a resonating surface as noise. The effect disappears mostly, as soon as the friction partners are separated by a lubricant.
However, dipping baths for zinc phosphating can be formulated in such a way that pre-rinsing without titanium phosphate nearly uniquely generates phosphophyllite (Fig. 7d and 7e). Pre-rinsing not only leads to grain refinement which halves the arithmetical mean deviation starting from 1.4 μm. It also causes the formation of hopeite, the quantitative composition of which in the phosphate coating is dependent on the concentration and duration of the pre-rinsing bath. As a consequence of the formation of hopeite, this coating also shows the undesired stick-slip features at press-in.
Zinc calcium phosphate coatings predominantly consist of fine-grained parascholzite crystallites when they are formed under standard conditions (Fig. 8a and 8b). A film thickness of 1 μm is already sufficient to obtain a closed film. Having noticeably low roughness parameters (0.1 μm for the arithmetical mean deviation and 2 μm for the maximum surface roughness), they are suitable as carrier coating for paints but stick-slip features also occur at the press-in process.
Phosphating instructions for zinc calcium phosphate coatings do not normally schedule surface activation by pre-rinsing with titanium phosphate suspension. On the one hand, fineness of grain is already achieved because of the formulation of the dipping bath, on the other hand, it is a fact that the zinc calcium phosphate coating reacts differently to additional nucleation. Under the terms of the test the titanium phosphate pre-rinsing provokes the formation of further crystalline phases. The crystallites grow in competition and form irregular shapes. The layers first exhibit a moderate coarsening of the surface which after 20 min lies one grade higher in the roughness parameters than with zinc calcium phosphate without pre-rinsing (1 μm for the arithmetical mean deviation Ra and 10 μm for the maximum surface roughness Rz). A layer with different crystallite sizes can be documented by scanning electron microscopy (Fig. 8c). Similar to the zinc phosphate coatings without pre-rinsing, sufficient steel surface is left uncovered during the first minutes of dipping and formation of the conversion coating respectively, which serves as an iron source for the formation of phosphophyllite crystals. After a dipping time of a few minutes the layers consist of granular parascholzite crystals as well as of tabular zinc phosphate hydrate and zinc iron phosphate crystals. X-ray diffraction experiments after 2, 10 and 20 min of dipping time (Fig. 9) show that iron containing phosphophyllite is dominant.
The bond stability of phosphate coating and base material is Slipobtained by chemical reaction via semivalences to the point of heteroepitaxial growth . The bond between coating and base material is therefore dependent on the chemical composition of the conversion coating. The different ratios of crystalline phases such as hopeite and scholzite and the iron containing phosphophyllite thus considerably influence the bond strength. Experience and experiments  have shown that the phosphophyllite ratio in the phosphate coating plays an important role in forming the phosphate pressing pattern (in plastic deformation) as well as in corrosion protection effects for paint systems. It is not astonishing that press-in tests for shaft-hub press-fit connections (see above) have positive results when a certain phosphophyllite ratio is available in the conversion coating.
5 Tribological properties of phosphated press-fit areas
Static press-in and press-out tests, static twisting tests as well as dynamic changing-torsion tests were carried out to evaluate the mechanical and tribological applicability of the phosphate coatings as loadtransfer element for forces and moments which are deposited on the press-fit areas of the press-fit connections. The static and dynamic load-transfer capability and the tribological behaviour (course of press-in force, stick-slip behaviour, tribo-corrosion, the danger of frictional fatigue fracture) were assessed. The bond stability of the coating was tested under severe conditions, e.g. through multiple press-in and press-out processes of the shaft and multiple static twisting of the shafts in the hubs. Dynamic changing-torsion tests predominantly served to evaluate the resistance to tribo-oxidation (frictional corrosion) and therefore also the danger of frictional fatigue fractures.
Press-in tests give a first impression of the applicability of the advanced phosphate coatings. Figure 10 shows the test system consisting of the phosphated shaft and the uncoated hub. Whereas, for example, a shaft coated with zinc phosphate could only incompletely be pressed-in into the hub (Fig. 10a), a manganese phosphated shaft can be pressedin completely under the use of the same force and without any stick-slip features (Fig. 10b). After the press-out the phosphate pressing pattern on the shaft is visible in Figure 10c. Whereas zinc phosphate coatings show strong stick-slip features to the point of complete abrasion of the conversion film, phosphophyllite containing zinc calcium phosphate coatings and manganese phosphate coatings form shiny well-preserved pressing pattern at press-in.
At press-in the phosphate coatings which exclusively consist of hopeite or scholzite crystals revealed unwanted stick-slip features. These can be avoided by using multiphase zinc calcium phosphate coatings (Fig. 11), especially through embedding iron containing phosphophyllite. In series of experiments with different dipping baths it was documented that on the one hand the positive properties of multiphase zinc calcium coatings depend on the quantitative composition of the phase ratio and that on the other hand this composition is not reproducibly adjustable for industrial applications with most of the commercial dipping baths available at present. To add to this, the composition of the steel to be coated and its surface treatment are further, not yet very well examined factors which influence the quality of the phosphate coating.
The application of manganese phosphate coatings helps avoid these difficulties. They prove their worth in tests under dynamic load, they are excellently suitable for the transmission of torques and prevent the formation of frictional corrosion (tribocorrosion). This is impressively documented by the results of the practical press-in and twist test with real component geometry (Fig. 12 and 13).
Fig. 12: longitudinal press-fit connection for torque transmission tests; uncoated hub (top left), phosphated shaft (top right), press-fit connection after 10,000,000 load alternations, one hub halve removed (bottom)
For testing the longitudinal press-fit connections under dynamic load (according to Gropp ), shafts with a jointing nominal diameter of 30 mm were pressed-in into hubs and loaded with changing torques. Torque transmission of the corresponding size leads to local sliding and without the phosphate coating consequently to the formation of frictional corrosion in the sliding areas. Zinc calcium phosphate coatings with zinc iron phosphate content as well as microcrystalline manganese phosphate coatings proved suitable. Figure 12 shows, for example, a longitudinal press-fit connection coated with manganese phosphate, the hub of which was cut open after the test. The well-pronounced front sliding area is clearly visible. The front and the back sliding areas are free from frictional corrosion even after more than 10,000,000 load alternations. A further advantage of phosphating turned out to be the high load-transfer capability of the longitudinal press-fit connection.
In this case, the stick-slip features visible in Figure 13a and 13b are irrelevant for the practical application since they lie far in excess of the realisable dynamic work load which is limited by the fatigue design strength of the shaft and the relevant safety.
During the tests it became apparent that inappropriately treated surfaces strongly change the phosphating behaviour. Figure 14 demonstrates the coarsened crystallisation of the conversion coating on the shafts which resulted from faulty grinding of the shaft surface. Sample sheet metals coated for checking purposes at the same time revealed reproducible microcrystalline phosphating. Coarse crystallites next to uncovered roughness peaks which developed on an unevenly ground surface led to strong stick-slip features.
Defects at the pressing pattern in the phosphate steel compound became visible after the press-out of unterthe bond. The image of damage (Fig. 15a) shows numerous cracks and separations in the pressing pattern. Apart from the vertical press-in direction the horizontal marks of the grinding direction are still observable. Cross sections show surface peaks between the phosphate-filled chatter marks, the phosphate coating of which was removed by the press-in process. (Fig. 15b and 15c).
Fig. 15: Damaged coating of a phosphated shaft; separations in the pressing pattern (horizontal: chatter marks, vertical: press-in direction) (a), cross section (middle: phosphate coating, left: shaft, right: embedding) (b), section of 15b; middle: phosphate-filled chatter marks (c)
6 Summary and conclusion
Commercial phosphating baths are mostly developed for microcrystalline single-phase coatings which can be precipitated with high reproducibility. The titanium phosphate pre-treatment of the steel surfaces promotes the formation of hopeite. This reaction leads to a deceleration of the covering process of the free surface and consequently to an increase in the amount of iron containing phosphophyllite . The different steel types also have an influence. Because of the different alloys and structures of the steel types they react at different rates to the pickling attack. More research needs to be conducted here.
Multiphase phosphate coatings containing phosphophyllite show improved tribological properties compared to zinc calcium phosphate coatings. This can be seen especially on the significantly decreased stick-slip inclination. From today’s point of view the use of manganese phosphate coatings is to be preferred for many press-fit connections because they can be reproducibly precipitated, guarantee a higher torque transmission und successfully prevent tribooxidation. This recommendation is especially based on the investigation described in this paper and on the micro-structural experiments as well as the results of the press-in tests and twist tests carried out on a technological scale.
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