Experimental researches concerning lapping of flat surfaces.

Cohal, Viorel ; Serb, Adrian ; Ciobanu, Romeo-Mihai 等

1. INTRODUCTION

Lapping consists in the final smoothing of previously grinding surfaces. This is done by means of certain fine abrasive particles impressed on the lap or freely interposed between the piece to be processed and the lap. Abrasive pastes applied on the tool may also be used. Through the relative motion of the lap compound to the piece, in the presence of abrasive grains, particles from the processed material are removed.

Lapping is necessary to obtain only special smoothness of the surface or to obtain extremely high dimensional precision together with special smoothness of the surface.

All lapping processes can be described as four component systems (a lap, a granule, a carrier fluid and a worckpiece) and that the mechanisms involved can be grasped by first understanding the interactions among those components. Lapping is differentiated technologically, but not in this mechanistic view; the relative size of the "grit" and the surface layer removed may change dramatically, but the processes all rely on interactions between these basic elements.

The object of lapping is to modify the workpiece. Workpieces vary in bulk chemical composition and may have property variations as functions of both lateral dimension and depth.

Depth variations may be intrinsic (due to different bonding for surface and bulk species) or may arise during the manufacture of the workpiece. They may arise from processing conditions (like plastic work of the near surface, or sub-surface cracking) or may be part of the lapping design process.

The fluid phase of the slurry may be characterized by its chemical composition and by its physical properties.

Chemical compositions of fluids include water and nonaqueous fluids like hydrocarbons and alcohols. The pH of the fluid may be controlled by addition of acids or bases, or by the use of a buffer system. In CMP, the fluid also contains a primary chemically active ingredient (like hydrogen peroxide or other oxidizers for metal) and may include secondary chemical ingredients (like inhibitors to control chemical interactions), or physically active additives (like surfactants).

Physical properties of the fluids affect both fluid dynamics and material transport in lapping. These properties include viscosity, density and thermal conductivity, all of which are pressure and temperature dependent. These properties can also be varied by changes in the chemical composition of the fluid.

Workpiece-fluid interactions involve both chemical and physical effects. The chemical effects include dissolution, etching and passivation, each of which change the workpiece surface in some way, and each of which has separate applications. Chemical changes in surface properties are sometimes described as "softening" of the workpiece, since they can make it more susceptible to lapping. Physical and mechanical effects involve transport of material and heat away from the workpiece surface during lapping; these processes are typically also affected by the nature of the lap and will be considered as three-way interactions below. All of these processes depend on the nature of the workpiece and the fluid.

Dissolution occurs when the chemical properties of the fluid and the workpiece surface match in specific ways. This match may hold for the bulk workpiece or only for its surface layer. In etching, a chemical reaction removes electrons from metals to produce soluble metal ions, which can then dissolve into the fluid (Evans & Paul, 2003).

Dissolution and etching are two-component methods of material removal. They are equivalent to lapping without either abrasive or lap, and are standard techniques in many applications which are not discussed here.

Passivation in particular, or coating in general, occurs when a chemical reaction between the workpiece and chemicals in the fluid results in a surface film that has different properties than the bulk workpiece. For metals, this film can reduce electrical or thermal conductivity, and is a desired or detested outcome depending on the particular application. For conventional lapping, the effects of this surface film are usually negligible. However, chemical mechanical lapping uses alternating cycles in which a surface film is chemically created and then mechanically removed. Thus chemical effects can be important. If the surface film is soluble, as is the case for most copper compounds, then this consideration must be included in describing any material removal process (Konig, 1990).

2. EXPERIMENTS AND RESULT OF RESEARCHES

The experiments have been carried out for cylindrical ([PHI]20 x 10) workpieces made by bronze (70 HV), brass (105 HV), annealed OLC45 steel (205 HV), 40Crl0 (220 HV) steel and bearings hardened steel RUL1 (700 HV). The lapping disk pressure was kept constant at 5 daN/[cm.sup.2], working time t = 3 min and cutting speeds values were 16, 32 and 44 m/min. A universal lapping paste was used composed of: white alundum--30%, vaseline oleum--47 % and stearin--23 %.

The experiments showed that together with the work hardness increasing to 200...220 HV, the material removal rate decreases significantly. For greater values of hardness, the decrease is slower (Cohal, 1998).

The surface roughness decreases when the hardness is smaller. When machining bronze and brass workpieces (with smaller hardness), the minimum roughness can be obtained for a cutting speed v=44 m/min. For the other materials, the optimum value for the cutting speed is v=32 m/min.

One of the most important cutting parameter is the disk pressure upon the work. Its great values influence positively the cutting capacity but the influence is opposite to the surface roughness.

[FIGURE 1 OMITTED]

Recommended values in case of lapping for this pressure are 0,4 ... 5 daN/[cm.sup.2]. The real machining pressure is much greater because of the machined surface which is much smaller than the workpiece surface and is continuously changing during machining. At the beginning, the disk-work contact is made only on the peaks of the surface roughness, the pressure is greater and the progress of machining is faster (approx. 3 urn/min in the case of steel). After a short time, the contact area increases and both pressure and cutting capacity are lower (approx. 1,5 [micro]m/min).

This result of researches was analysis with NCSS (fig.1). Number Cruncher Statistical System (NCSS) is an advanced, easy-to-use statistical analysis software package. Regression techniques analyze the relationship between a dependent (Y) variable and one or more independent (X) variables. NCSS has regression procedures for many different situations

The assumptions of the one-way analysis of variance (fig.1.) are:

1. The data are continuous (not discrete).

2. The data follow the normal probability distribution. Each group is normally distributed about the group mean.

3. The variances of the populations are equal.

4. The groups are independent. There is no relationship among the individuals in one group as compared to another.

5. Each group is a simple random sample from its population. Each individual in the population has an equal probability of being selected in the sample.

The assumptions of the Kruskal-Wallis test are:

1. The variable of interest is continuous (not discrete). The measurement scale is at least ordinal.

2. The probability distributions of the populations are identical, except for location. Hence, we still require that the population variances are equal.

3. The groups are independent.

4. All groups are simple random samples from their respective populations. Each individual in the population has an equal probability of being selected in the sample.

There are few limitations when using these tests. Sample sizes may range from a few to several hundred. If your data are discrete with at least five unique values, you can assume that you have met the continuous variable assumption. Perhaps the greatest restriction is that your data come from a random sample of the population. If you do not have a random sample, your significance levels will be incorrect (Hintze, 2007).

Given that the analysis of variance (ANOVA) test finds a significant difference among treatment means, the next task is to determine which treatments are different. Multiple comparison procedures (MCPs) are methods that pinpoint which treatments are different.

If the F-test from an ANOVA for this experiment is significant, we do not know which of the three possible pairs of groups are different. MCPs can help solve this dilemma.

3. CONCLUSIONS

Lapping pressure and friction lead to a decreasing of the abrasive grains dimensions. Consequence of this, the cutting capacity decreases and the surface roughness is better. This is why there are recommendations to start the cutting process with sharp grains and to continue with worn ones.

Lapping is successfully applied in fine mechanics manufacturing, increasing the cutting tools lifetime and precision and also in finishing and superfinishing the ceramic workpieces.

4. REFERENCES

Cohal, V., Contributions about surfaces lapping. Doctor's degree thesis. Iasi, Romania, 1998

Evans, C.J., Paul, E., Material Removal Mechanisms in Lapping , Annals of the CIRP Vol. 52/2/2003

Hintze, J., NCSS Quick Start & Self Help Manual, Kaysville, Utah, USA, 2007.

Konig, W., Finish Processing, Ceramic Materials, Zunch, 1990