Aus der Praxis für die Praxis 1 The lapping process Lapping is a high precision machining process which allows creating not only very high surface quality but also dimensional accuracy and narrow size tolerances of the machined work pieces. It can be used for nearly all materials which do not plastically deform under load. Even very hard materials like glass, ceramics or cemented carbides can be machined by lapping. During the lapping process the surface of the work piece and an opposite surface are rubbed together with a lapping lubricant in the working gap between them. The metal removal is generated by abrasive lapping powders which are not bound in a tool surface but are dispersed in the lapping lubricant. The abrasive grains of the lapping powder are rolling and sliding on the work piece surface. They are primarily performing a kind of kneading procedure causing micro cracks which lead through crack-linking to a break out of small particles and finally results in very smooth, planar surfaces (figure 1). Few grains also cut small chips out of the surface. Therefore lapping is not a regular cutting process like grinding Tribologie + Schmierungstechnik 64. Jahrgang 6/ 2017 29 * Dipl.-Ing. Wilhelm Rehbein LANXESS Deutschland GmbH Lubricant Additives Business, Mannheim, Germany Influence of Lubricant Additives on Surface Quality and Material Removal Rate in a Lapping Process W. Rehbein* Figure 1: grains of lapping powder in a lapping process. Source: Bierther Submicron GmbH, Wikipedia Einfluss von Schmierstoffadditiven auf die Oberflächenqualität und die Abtragsgeschwindigkeit beim Läppen Läppen ist ein Präzisions-Bearbeitungsprozess, bei dem die Oberfläche eines Werkstücks und eine gegenüberliegende Oberfläche aufeinander gerieben werden. Im Arbeitsspalt zwischen den Oberflächen befindet sich das Läppmedium. Mit diesem Bearbeitungsverfahren werden sehr glatte, ebene Flächen hergestellt. Der Materialabtrag wird durch abrasive Läpppulver erzeugt, die nicht in einer Werkzeugoberfläche gebunden sind, sondern im Läppöl dispergiert werden. Durch Auswahl geeigneter Schmierstoffadditive ist es möglich, die Eigenschaften des Läppöls an die spezifischen Anforderungen des Läppprozesses anzupassen. Basierend auf Laborprüfungen und Feldtestergebnissen beschreibt der Beitrag die Eigenschaften verschiedener Additivtypen und deren Einfluss auf die Parameter des Läppprozesses. Schlüsselwörter Läppen, Läppmittel, Läppöl, Oberflächenqualität, Zeitspanungsvolumen, Abtragsrate Lapping is a metal removal process which is used to produce planar surfaces with very low surface roughness. The metal removal is generated by abrasive grains which are not bound in a tool surface but are dispersed in a special lapping lubricant. By a selection of suitable lubricant additives it is possible to adjust the characteristics of the lapping oil to the specific demands of the lapping process. Based on laboratory test methods as well as field test results the paper describes in detail the properties of different types of additives and their influence on the listed parameters. Keywords Lapping, lapping agent, lapping oil, surface quality, material removal rate Kurzfassung Abstract T+S_6_17 16.10.17 10: 40 Seite 29 Aus der Praxis für die Praxis or honing but mainly a process of removing material by fatigue. For starting the lapping process the work pieces are placed with the side which has to be machined facing the lapping plate. The lapping plate is a disk on the lapping machine, usually provided with radial slots to drain the used lapping medium and removed work piece material off the surface. It is usually made of hardened cast iron. The lapping plate is rotating and carries conditioning rings which rotate with the plate as a kind of planetary work drive system. By this way the direction of rotation for every single work piece is changing continuously (figure 2). This results in almost exclusively nondirectional machining marks and a matt-finished appearance of the lapped surface. By the means of pressure plates the work pieces are pressed on to the lapping plate. This pressure is usually kept low at the beginning of the process and increased after a while to achieve a higher material removal rate (MRR). Towards the end the pressure is reduced again to optimize the roughness of the lapped surfaces. In some cases the work pieces are covered by a second lapping plate (double-wheel lapping process). Lapping powders are abrasive grains of silicon carbide, aluminum oxide, boron carbide or even diamond powder. There are huge differences in the quality of these powders relating to particle size distribution, hardness, shape and number of edges. The particle size of the grains influences the surface finish of the machined work pieces as well as the achievable material removal rate. For preparing the lapping medium, the lapping powder is mixed with the lapping oil just before application. The mixing ratios range from 80 to 120 g of lapping powder to 1 litre of lapping oil for roughing and 65 to 80 g/ l for finishing processes. The lapping medium is usually continually stirred in a special mixing tank to avoid the separation of oil and powder. 2 The lapping oil The lapping oil does not only work as a carrier for the lapping powder. It also enables an even distribution of the abrasive lapping powder over the whole lapping plate. Additionally it prevents the formation of grooves and scratches on the work piece surface and reduces the temperature in the process by its lubricating and cooling properties. The transport of the removed material into the slots of the lapping plate or to the edge of the plate is another function of the lapping oil (figure 3). The viscosity of the lapping oil is very important for the efficiency of the process. If the oil is too viscous, the lubricant film between lapping plate and work piece is very thick and the lapping powder may lose the contact to the surfaces which have to be machined. This will significantly reduce the MRR. In case the viscosity is too low, the gap between the surfaces will be too small to enable the rolling of the lapping powder. This leads to grooves and scratches on the work piece. Usually the viscosity of the lapping oil is adjusted to the grain size of the lapping powder: larger grains require a higher viscosity, smaller grains a lower viscosity of the oil. To ensure a high quality of the machined surface with low surface roughness and also a high productivity of the lapping process, the lapping oil has to fulfil several requirements: • good dispersing capacity for the lapping powder to prevent fast sedimentation • prevention of agglomeration of the lapping powder • good wetting properties for a fast and easy preparation of the lapping medium and to achieve its homogeneous distribution on the lapping plate 30 Tribologie + Schmierungstechnik 64. Jahrgang 6/ 2017 Figure 2: lapping plate with conditioning rings Figure 3: a lapping plate covered by a homogeneous film of lapping medium T+S_6_17 16.10.17 10: 40 Seite 30 Aus der Praxis für die Praxis • sufficient lubricating properties to enable the rolling of the lapping powder • optionally anticorrosion properties to prevent flash rust on the machined work pieces • compatibility with work piece materials and machine 2.1 Dispersing capacity The lapping powder should mix easily with the lapping oil and should remain suspended for some time before settling. The slow settling is important to avoid a separation in the tubes or on the lapping plate which would cause an inhomogeneous surface finish on the lapped work pieces. Usually the mixed lapping medium is continually stirred to avoid separation however in case the stirrer is stopped for some time the sedimented powder should easily remix again. It is possible to reduce the settling speed by choosing higher viscous lapping oil however this usually has a negative influence on the material removal rate and, because of poorer flushing properties, also on the achievable surface quality. 2.2 Prevention of agglomeration When the lapping powder conglomerates in the oil the resulting agglomerates have the multiple size of the single particles. Therefore these agglomerates do not roll in the lapping process but stick in the gap between the lapping plate and the work piece. Carried by the motion of the lapping plate they are able to cause broad and deep grooves (figure 4) on the surface of the work piece. Depending on the kind of lapping powder the additives of the lapping oil have to be carefully selected to avoid an increased agglomeration, separation or sedimentation. In particular high polar additives like some organic acids are able to form concrete like sediments which are hardly redispersible. The agglomeration of the lapping powder (figure 5) is caused by the attractive London-van der Waals forces occurring between the particles. In order to stabilize the lapping medium against agglomeration and fast separation, repelling forces must be established between the particles by adding deterging / dispersing additives which adsorb on the particle surface. These detergents / dispersants keep the particles at a distance and reduce the tendency towards agglomeration by means of electrostatic repulsion and/ or steric stabilization. Among the many types of detergents / dispersants which are used as additives for lubricants calcium sulfonates showed a very good dispersing efficiency towards the different types of lapping powders. By adding calcium sulfonates it was possible to avoid the formation of lapping powder agglomerates. Also the settling time of the grains was extended. The sediment which was formed was soft and easy to redisperse even after some days. Because they furthermore work as corrosion inhibitors they are additionally able to prevent the machined work pieces from flash rust. 2.3 Wetting properties For fast and homogeneous preparation of the lapping medium the air and moisture at the particle surface has to be displaced and replaced by the lapping oil. Additionally the lapping medium has to form an even and consistent film on the lapping plate. Surface active agents increase the wetting properties of the lapping oil. The lapping medium spreads more homogeneously on the lapping plate and work piece surfaces providing a constant film thickness over the complete surface and avoiding the accumulation of particles. 2.4 Lubricating properties If the lubrication of the lapping oil is not sufficient the abrasive grains of the lapping powder will not roll in the Tribologie + Schmierungstechnik 64. Jahrgang 6/ 2017 31 Figure 4: grooves on a lapped surface, caused by agglomerated particles of the lapping powder Figure 5: agglomeration of lapping powder in inappropriate lapping oil T+S_6_17 16.10.17 10: 40 Seite 31 Aus der Praxis für die Praxis gap between the surfaces of the lapping plate and the work piece. They just slide and rub over the surfaces causing scratches which damage the surface finish (figure 6). This effect can be avoided by increasing the viscosity of the lapping oil. An increased oil viscosity however will lead to an increased thickness of the oil film. Thus the smaller particles of the lapping powder will lose the contact to at least one of the surfaces which will cause a reduced material removal rate. By using suitable additives the lubricating properties can be optimized without increasing the oil viscosity. Additives which are able to improve the lubricity are usually polar organic substances containing heteroatoms like oxygen, nitrogen or other atoms with electronegativity higher than carbon or hydrogen. By physical adsorption of these molecules a very thin layer is formed on the metal surface which effectively reduces the friction between surfaces moving relative to each other. 2.5 Corrosion inhibitors Depending on the kind of metal which is machined the new generated surfaces can be very sensitive towards corrosion. Even when stored indoors flash rust may appear after some days. This happens much faster in a high humidity atmosphere. To prevent corrosion a corrosion inhibitor which is compatible to the other additives in the formulation can be added. 3 Field test results To evaluate the laboratory test results field tests were carried out on a 1-wheel flat lapping machine with a 1000 mm lapping plate which rotated with a revolution of 50 rpm. The ring shaped work pieces were made of cast iron with diameters of approximately 200 mm, the lapped surface was 1.6 mm broad. Silicon carbide with an average particle size of 12 µm was used as lapping powder in a concentration of 100 g per litre of lapping oil. The base oil which was used for the lapping oil was a group I paraffinic mineral oil with a kinematic viscosity of 25 mm 2 / s at 40 °C. The lapping medium was prepared in a mixing vessel which was equipped with a stirrer which worked continuously. After the lapping process, which lasted 20 minutes, the test work pieces were cleaned and their surface roughness and structure was observed. The evaluation of the test was done in comparison to a commercially available lapping oil which was used as reference. The following parameters were used: • R max : maximum roughness depth, the largest vertical distance between the highest peak and the deepest valley of a surface profile within the evaluation length. • MRR: material removal rate, the volume of material removed divided by the machining time. For the first test a combination of calcium sulfonates (DP 1) was used to ensure an optimal dispersion and to avoid the agglomeration of the silicon carbide grains. The fine distribution of the abrasive grains resulted in an improve-ment (reduction) of the R max value by 19 % (figure 7). However the MRR was also decreased by 12 % and some small scratches were detected on the lapped surface 32 Tribologie + Schmierungstechnik 64. Jahrgang 6/ 2017 Figure 6: small scratches on a lapped surface, caused by abrasive grains which are not able to roll due to insufficient lubrication ! "#$% ! "! &% ! "&'% ! "#% ("! )% *++% $$% ,)% *+&% *! *% )+% #+% $+% ,+% *++% **+% *! +% *(+% *"&% *"#% *",% ! "*% ! "(% ! "&% ! "#% ! ",% ("*% ("(% ("&% -./ .0.12.% 34%*% 34%! % 34%(% 34%'% ! "#$%&"'(%$)*+"'(%"#$(,-.( / )"0 (,1).( -567% 8--% Figure 7: maximum roughness values and material removal rates of the tested lapping oils T+S_6_17 16.10.17 10: 40 Seite 32 Aus der Praxis für die Praxis which indicated an insufficient lubricity of the oil formulation. To examine this effect we compared the reference oil and the formulation DP 1 by using a SRV high frequency tribometer. When compared to the reference oil, DP1 showed a significantly higher coefficient of friction (COF) in this test (figure 8). This indicated that the calcium sulfonates have quite good dispersing however very limited lubricating properties. For the second test (DP 2) the calcium sulfonates were combined with an additive based on special sulfurized esters and hydrocarbons. This additive has a highly polar molecular structure which allows it to adsorb physically on metal surfaces forming a shear stable friction reducing layer. The SRV test showed a strongly reduced COF for this formulation. Because of the active sulfur content it was also expected that the MRR would be increased. Due to the improved lubricity the surface inspection of the work pieces which were machined with formulation DP 2 didn’t show any more scratches. Also the MRR was increased up to approximately 96 % compared to the reference whilst R max still was 8.6 % lower. In a further lapping oil formulation (DP 3) a sulfurized olefin with very high active sulfur content was added to the combination of calcium sulfonates and special sulfur carrier. This was done to examine the influence of active sulfur on the lapping process. The COF determined by the SRV test was still lower than the reference oil. Active sulfur is defined according to ASTM D-1662 as the part of sulfur in an additive or base oil which reacts with copper powder within 1 h at 149 °C. The amount of active sulfur in a sulfurized additive is influenced by the length of the sulfur bridge connecting the hydrocarbon respective ester parts of the molecule. With increasing length the sulfur carrier gets more active, i. e. a sulfur bridge consisting of 1 or 2 sulfur atoms is quite inactive at 149 °C whereas a bridge of 4 to 5 sulfur atoms is very active. The active sulfur chemically reacts under suitable conditions with metal surfaces which finally results in the formation of metal sulfide layers (figure 9). Because of their ionic bonds, these layers have a much lower elasticity and plasticity than the original metal. Under the conditions of a lapping process where the rolling lapping powder causes a constant elastically and plastically deformation of the work piece surface it was expected that the more brittle metal sulfide would break easy into small particles which would increase the MRR. As estimated, the material removal rate was strongly increased to 105 % by the addition of the active sulfurized olefin. The surface roughness R max was approximately 3 % lower than the reference oil. Again the lapped surfaces were free from scratches. An increase of the concentration of the active sulfurized additive (DP 4) led to a further rise of the MRR to 121 % of the reference. However the quality of the lapped surface was significantly decreased. The higher treat rate of the active sulfurized olefin obviously caused an increased formation of metal sulfides which are sheared off by the lapping powder. In case a thicker layer of metal sulfides Tribologie + Schmierungstechnik 64. Jahrgang 6/ 2017 33 ! " ! #! $" ! #! %" ! #! &" ! #! '" ! #(" ! #($" ! #(%" ! #(&" ! #('" ! #$" ! " $! ! " %! ! " &! ! " '! ! " (! ! ! " ($! ! " ! "#$! %#&'(")()*%! +"&( +,#(-.#! "&/ .0( )*+*,*-.*" / 0"(" / 0"$" / 0"1" Figure 8: coefficients of friction of the lapping oils, measured by SRV tribometer Figure 9: physical adsorption and chemical reaction of sulfurized additives T+S_6_17 16.10.17 10: 40 Seite 33 Aus der Praxis für die Praxis was formed also the particles which cracked out of the surface could be bigger leaving a more irregular surface profile with higher R max values. Contrary to the commercially available lapping oil there was no formation of flash rust observed on the lapped parts which were machined by using the formulations DP1 to DP4. 4 Conclusion A very high surface quality regarding roughness and groove formation of lapped work pieces can be achieved by using detergents / dispersants which prevent the agglomeration of the lapping powder. Also the settling speed and sedimentation of the lapping powder is positively influenced by detergents / dispersants. The best results were obtained by a mixture of calcium sulfonates with medium high TBN. They additionally work as corrosion inhibitors and prevent flash rust. The formation of small scratches on a lapped surface is caused by abrasive grains which are not able to roll due to insufficient lubrication. Polar lubricant additives can improve the lubrication by physical adsorption on the metal surfaces of the work pieces and thus prevent the arising of scratches. Active sulfur carriers like sulfurized olefins can be used to improve the material removal rate for a higher productivity of the lapping process. However the maximum roughness depth of the lapped surface rises as well. The expected mode of action for this effect is based on the formation of metal sulfides which easily break out of the work piece surface by the kneading effect of the lapping powder grains. By changing the treat rate of the active sulfurized olefin it is possible to adapt the formulation of the lapping oil to the specific requirements of the manufacturing process regarding achievable surface roughness and material removal rate. 34 Tribologie + Schmierungstechnik 64. Jahrgang 6/ 2017 Falls Sie eine Veröffentlichung wünschen, bitten wir Sie, uns die Daten auf einer CD, zur Sicherheit aber auch als Ausdruck, zur Verfügung zu stellen. Schön ist es ferner, wenn die Bilder durchnummeriert und bereits an der richtigen Stelle platziert sowie mit den zugehörigen Bildunterschriften versehen sind. Da wir auf die Einheit von Text und Bild großen Wert legen, bitten wir, im Text an geeigneter Stelle einen sogenannten (fetten) Bildhinweis zu bringen. Das Gleiche gilt für Tabellen. Auch sollten die Tabellen unsere Art des Tabellenkopfes haben. Die Artikel dieses Heftes zeigen Ihnen, wie wir uns den Aufbau Ihres Artikels vorstellen. Vielen Dank. 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